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Jan 28 2009
Reposted with permission from Robert Nyman:
Admit that you have always wanted to know how to develop a Firefox
extension but never had the time to learn. Here I will walk you through
and at the end of the article we will have created a fully functional Firefox
extension!
We will create a Firefox extension to find all links in the current web page,
highlight those which have a target attribute and alert you how many
links it found. The good part is that once you have done this, you have
both an understanding of Firefox extension development as well as a
blueprint for any extension you would want to develop in the future.
First, let’s start with setting up your development environment. You need
Firefox (duh) and basically whatever code editor you prefer. Then, there
are some recommended things to do to prepare Firefox:
The first step is to create a different profile in Firefox, since you will do
some settings and changes that you probably don’t want for your regular
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profile. In my case, I’ve created a new development profile named “dev”.
The steps to do this are:
Open the Windows Start menu and choose the Run option (on Vista, it
might not be there – just press Windows key + R in that case). In the run
dialog, write firefox -P and press enter/click OK. Choose Create
Profile in the dialog and follow the steps.
Open the Terminal (located under /Applications/Utilities) and type in
/Applications/Firefox.app/Contents/MacOS/firefox
-profilemanager. Choose Create Profile in the dialog and follow the
steps.
Open a terminal, use CD to navigate to your Firefox directory and then
enter ./firefox -profilemanager. Choose Create Profile in the
dialog and follow the steps.
Open Firefox through the Profile Manager (process described above, or set
the development profile as default during extension development). Then
enter about:config in the address bar. It will warn you about changing
settings, but it’s ok since what you will do is only minor changes for
development. You can filter the existing settings, and if any of the below
settings don’t exist, you can just create them.
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These are good to enable extension errors in the Firefox Error Console
(Tools > Error Console), disable XUL caching and such.
javascript.options.showInConsole = true
nglayout.debug.disable_xul_cache = true
browser.dom.window.dump.enabled = true
These aren’t necessary, but they might help you out. Personally, I don’t
use these.
javascript.options.strict = true
extensions.logging.enabled = true
Instead of constantly preparing and reinstalling your extension, there’s a
simple way to add a pointer from your Firefox extensions directory to your
code location. To do this, you must first find your profile directory:
The profile directory is where you will find all the settings for your Firefox
profiles, including extension information.
In Windows 2000 and XP, open Explorer and go to C:\Documents and
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Settings\[your user name]\Application Data\Mozilla
\Firefox\Profiles and in Vista, go to C:\Users\[your user
name]\AppData\Roaming.
Open the Terminal and type in CD ~/Library/Application\
Support/Firefox/profiles/. There you will find your Firefox
profiles, and they will be named with letters and numbers, followed by a
dot (.) and then your profile name, e.g. 12a3bc4d.dev.
Open a terminal and type in CD ~/.mozilla/. In that location, you will
find all your Firefox profiles, and they will be named with letters and
numbers, followed by a dot (.) and then your profile name, e.g.
12a3bc4d.dev.
In your development profile folder, you will find a folder named
extensions. In it, you will have code for all your installed extensions.
Instead of placing your code there, you can create a pointer file. Do that by
creating a file with a unique name for you (this will have to be the same as
you chose for your em:id value in your install.rdf file – more on that
below).
In the case of our example, create a file named
[email protected] , without any extension, and in
it just point it to where you will have your code, e.g. C:\extensions\
(Windows) or ~/Sites/linktargetfinder/ (Mac).
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What is needed to have a good base for your extension development, is to
create the structure of the extension code. Start by creating this hierarchy:
We begin with the intimidating code of install.rdf. This is where you
will have all the meta information about your extension, which versions of
Firefox it supports and other assorted information. Our install.rdf
will look like this:
<?xml version="1.0"?>
<RDF xmlns="http://www.w3.org/1999/02/22-RDF-syntax-ns
xmlns:em="http://www.mozilla.org/2004/em-RDF#">
<Description about="urn:mozilla:install-manife
<em:id>[email protected]
<em:name>Link Target Finder</em:name>
<em:version>1.0</em:version>
<em:type>2</em:type>
<em:creator>Robert Nyman</em:creator>
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<em:description>Finds links that have
<em:homepageURL>http://www.robertnyman
<em:optionsURL>chrome://linktargetfind
<em:targetApplication>
<Description>
<em:id>{ec8030f7-c20a-
<em:minVersion>2.0</em
<em:maxVersion>3.1b2</
</Description>
</em:targetApplication>
</Description>
</RDF>
Scary, right? Well, even if your not used to RDF it’s not that bad at all.
Like most code, there’s some mandatory information that you will never
need to change, and then some of your own to sprinkle in. The interesting
parts for us are:
em:id
This is where you add your unique developer id, of your own
choosing. Note that this has to be the same as the pointer file you
created above.
em:name
The name of your extension.
em:version
Current version of your extension.
em:type
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The type declares that is an extension, as opposed to, for instance, a
theme.
em:creator
Well, you!
em:description
Describes your extension functionality. Will be shown in the Tools >
Add-ons window.
em:homepageURL
The URL of your extension’s web site.
em:optionsURL
The URL to where you will have your file for editing
options/preferences.
em:id
This value is the actual id of Firefox. Change this if you want to
develop for Thunderbird or something else.
em:minVersion
The minimum version of Firefox required to run the extension. Valid
Application Versions.
em:maxVersion
The maximum version of Firefox required to run the extension. Valid
Application Versions.
Read about more options in Install Manifests.
The chrome of Firefox is everything around the content window. i.e. web
browser toolbar, menus, statusbar etc. The next file for our extension,
which will probably feel a bit awkward to edit, is the chrome.mainfest
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file. This one, however, is in conjunction with install.rdf the key to
how your extension will be added to Firefox, and how it will work. Our
chrome.manifest file looks like this:
content linktargetfinder chrome/content/
content linktargetfinder chrome/content/ conten
overlay chrome://browser/content/browser.xul chrome://
locale linktargetfinder en-US locale/en-US/
skin linktargetfinder classic/1.0 skin/
style chrome://global/content/customizeToolbar.xul
So, what are all those options? Let’s go through them:
content linktargetfinder chrome/content/
The path to where your extension content files will be found.
content linktargetfinder chrome/content/
contentaccessible=yes
Same as the above, but when contentaccessible=yes is added, it
allows Firefox 3 and later to access the extension’s files and show
them in the web browser (like within a web page). Found this
excellent help through Web pages accessing chrome:// is forbidden.
overlay chrome://browser/content/browser.xul
chrome://linktargetfinder/content/browser.xul
The path to which file you will use to override web browser elements,
and add items to the toolbar, menu and statusbar.
locale linktargetfinder en-US locale/en-US/
Used for localization of content.
skin linktargetfinder classic/1.0 skin/
Skin reference.
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style chrome://global/content/customizeToolbar.xul
chrome://linktargetfinder/skin/skin.css
Style overlays for chrome pages.
More information can be found in Chrome Manifest.
Ok, once the mandatory parts are out of the way, now things start to get
interesting. This is also when we start to look at XUL, which stands for
XML User Interface Language. It is developed by Mozilla to create
interfaces in Firefox, Thunderbird etc.
First, within the chrome/content folder, create three files:
browser.xul
options.xul
linkTargetFinder.js
This is the file we will use to override some of the default look of the web
browser, i.e. add a button to the toolbar, an item to the Tools menu and a
statusbar icon. Let’s look at the complete browser.xul file and then break it
down:
<?xml version="1.0"?>
<?xml-stylesheet href="chrome://linktargetfinder/skin/
<!DOCTYPE linktargetfinder SYSTEM "chrome://linktarget
<overlay id="sample" xmlns="http://www.mozilla.org/key
<script src="linkTargetFinder.js" />
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<menupopup id="menu_ToolsPopup">
<menuitem label="&runlinktarge
</menupopup>
<keyset>
<key id="link-target-finder-ru
</keyset>
<statusbar id="status-bar">
<statusbarpanel id="link-targe
</statusbar>
<toolbarpalette id="BrowserToolbarPale
<toolbarbutton id="link-target
</toolbarpalette>
</overlay>
First, there some general XML syntax, then you include your CSS code
and set a DOCTYPE for localization. It’s followed by the overlay root
element with its Ghostbuster-inspired namespace (developers are truly
geeks ).
After that you’re free to write whatever XUL you want, and basically it’s
like HTML should’ve been. What I mean with that is that it is filled with
basic interface options that can’t be found anywhere in HTML. For a
complete overview of what you can do, please read the XUL Reference.
In our code here, we start with adding a menu option to the Tools menu in
Firefox, and connect it to a keyboard shortcut:
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<menupopup id="menu_ToolsPopup">
<menuitem label="&runlinktargetfinder;" key="l
</menupopup>
<keyset>
<key id="link-target-finder-run-key" modifiers
</keyset>
Followed by adding an icon to the Firefox statusbar:
<statusbar id="status-bar">
<statusbarpanel id="link-target-finder-status-
</statusbar>
And at the end, we add a button to the Firefox toolbar:
<toolbarpalette id="BrowserToolbarPalette">
<toolbarbutton id="link-target-finder-toolbar-
</toolbarpalette>
Note that you need to go to View > Toolbars > Customize… to add your
button to the visible toolbar.
In the above examples, you might have noticed the
&runlinktargetfinder; code. That is used for localizing text, and its
translation can be found in the locale folder. More on that later.
Another thing to notice, which can be tricky at first, is that some ids and
classes for XUL elements actually have meaning for its display, stemming
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from core Firefox code. Make sure to read the XUL Reference thoroughly
for the elements you’re using.
If you want to use regular HTML elements instead of/together with XUL,
you can include the XHTML definition in your root element, and then
include any HTML element with the html: prefix. Like this:
<overlay id="sample" xmlns="http://www.mozilla.org/key
<html:input type="submit" value="Send" />
This file is used for the options/preferences dialog for your extension, and
its path is pointed out in the install.rdf file in the
Description/em:optionsURL node. The complete options file for our
extension looks like this:
<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="te
<prefwindow
title="Link Target Finder Preferences"
xmlns="http://www.mozilla.org/keymaster/gatekeepe
<prefpane label="Link Target Finder Preference
<preferences>
<preference id="link-target-fi
</preferences>
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<groupbox>
<caption label="Settings"/>
<grid>
<columns>
<column flex="
<column flex="
</columns>
<rows>
<row>
<label
<check
</row>
</rows>
</grid>
</groupbox>
</prefpane>
</prefwindow>
Again, some regular XML information and styling of the content. It then
uses some XUL to layout the content of the preferences dialog. The
interesting part here, though, is connecting the user input to the actual
settings for the extension (those settings are to be found in the prefs.js
file, mentioned in detail below).
To start with, you create a preferences group in the options.xul file
with your desired preferences. Then, give each preference an id and
connect it to the extension’s real preferences with the name attribute:
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<preferences>
<preference id="link-target-finder-autorun" na
</preferences>
Then you create an element and add a preference attribute to it,
pointing to the before-chosen idattribute for the preference in the
preferences group. It will then automatically connect its value to that
specific preference.
<checkbox id="autorun" preference="link-target-finder-
Note that you don’t need to add an OK button for the dialog – for instance,
on Macs there won’t be one while it’s automatically added for Windows
users. All about system compatibility, my friends.
If you’re like me, this is where you will feel at home. Good ol’ JavaScript
code.
What’s nice here is that you can script any element in the XUL and any
element in the HTML content window as well. All you need to access the
content window is to precede your document call with the content
keyword, like this:
content.document.getElementsByTagName("a");
Here’s the complete code of the linkTargetFinder.js file:
var linkTargetFinder = function () {
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var prefManager = Components.classes["@mozilla
return {
init : function () {
gBrowser.addEventListener("loa
var autoRun = prefMana
if (autoRun) {
linkTargetFind
}
}, false);
},
run : function () {
var head = content.document.ge
style = content.docume
allLinks = content.doc
foundLinks = 0;
if (!style) {
style = content.docume
style.id = "link-targe
style.type = "text/css
style.rel = "styleshee
style.href = "chrome:/
head.appendChild(style
}
for (var i=0, il=allLinks.leng
elm = allLinks[i];
if (elm.getAttribute("
elm.className
foundLinks++;
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}
}
if (foundLinks === 0) {
alert("No links found
}
else {
alert("Found " + found
}
}
};
}();
window.addEventListener("load", linkTargetFinder.init,
While it’s a fair bit of code, it is also very basic. When the window loads, it
runs the init method of the linkTargetFinder object. If the
preference autorun is set to true, it calls its run method immediately.
Otherwise, it will only be called when the toolbar button, menu item or
statusbar icon is clicked. This happens through the oncommand attribute
on the elements in the browser.xul file.
The code in the run method is pretty straight-forward. It adds a CSS file
from the extensions chrome folder to the current document, finds all links
in it, loops through them and checks if they have a target attribute,
counts those, highlights them and alerts you with the number of hits.
As you can see, there’s a pointer in the code to something called
gBrowser. That is how to get a reference to the current web browser, and
you could also use getBrowser() as well. Note that this sort of code is
only available from within the XUL context of the web browser. More
information and options can be found in Tabbed Browser.
The only unusual part for a JavaScript is the variable prefManager,
which connects to Firefox preference manager, and later gets the
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autorun preference with the help of this code:
var autoRun = prefManager.getBoolPref("extensions.link
The three types of extension preferences are string, integer and boolean,
and the six methods to work with them are:
getBoolPref()
setBoolPref()
getCharPref()
setCharPref()
getIntPref()
setIntPref()
Within the defaults folder, you create a preferences folder and in
that one a file named pref.js. This is used for the preferences you want
to use for your extension, and looks like this:
pref("extensions.linktargetfinder.autorun", false);
Used for localization. In our case here, we just have one child folder for
en-US content, but it can be easily extended. That folder has a file named
translations.dtd, which contains translations for entities used in
XUL files. Our file looks like this:
<!ENTITY runlinktargetfinder "Run Link Target Finder">
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Remember &runlinktargetfinder; in the browser.xul file? This is
where it gets its translation.
Now when we have all functionality in place, let’s make things a little more
pretty (just a little, ok?). In our skin folder, we have three files:
skin.css, status-bar.png and toolbar-large.png. The images
are, naturally, used for the toolbar and statusbar respectively.
The CSS in skin.css is used for setting the image for the toolbar, the size
and space for the statusbar icon and the highlight look of links with a
target attribute. The code looks like this:
#link-target-finder-toolbar-button {
list-style-image: URL("chrome://linktargetfind
}
#link-target-finder-status-bar-icon {
width: 83px;
margin: 0 5px;
}
.link-target-finder-selected {
outline: 2px solid red !important;
}
Firefox extensions are delivered like XPI files, and those are basically just
ZIP files with another extension. Therefore, when you’re finished with
your extension, all you need to do is ZIP your files together and give it an
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XPI extension. Note: do not ZIP the containing folder for your extension,
just its contents (chrome folder, chrome.manifest and install.rdf
files etc).
Once you have your XPI file, you can just drag and drop it into Firefox,
and it will automatically install.
Select all the contents of your extension folder, right-click and choose
Send To > Compressed (Zipped) Folder. Rename it to .xpi
instead of .zip and you’re done!
Open the Terminal, navigate to your extension with the CD command and
then type in zip -r LinkTargetFinder.xpi *. Ta daa!
Open a terminal, get to your extension folder, and type in zip -r
LinkTargetFinder.xpi * All done!
There are two options of distributing your extension. Either use
addons.mozilla.org or host it yourself. If you do it yourself, you can specify
an updateURL in your install.rdf file, but note that since Firefox 3 it
has to be a secure location, e.g. through SSL or similar.
Personally, I’d recommend addons.mozilla.org for greater reach,
auto-update pushing and statistics. The downside of it, though, is that the
review process can take quite some time for your extension to get an
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30
approval. Before that it can still be downloaded as an experimental
add-on, but people need to have an addons.mozilla account and log in to
do so.
To make sure it doesn’t take longer than necessary, I recommend reading
Successfully Getting your Addon Reviewed.
All the code I’ve created here is available both as a ZIP file of everything,
so you can start experimenting, change stuff and see what happens. It is
also available as an XPI file, which you can install in your Firefox just to
test the functionality (drag and drop it into Firefox).
Download Link Target Finder code as a ZIP file.
Download Link Target Finder as an XPI file.
I hope this has been a good, albeit long, introduction to Firefox extension
development and that it has helped you to grasp concepts you hadn’t
gotten full control over yet. Good luck, and don’t hesitate to ask if you
have any questions!
Tags: documentation, tutorials
Categories: developers, documentation
Mattias Hising
There were a lot of discussions about developing your extensions a couple of years
ago, and somewhere the know-how and tutorials have gone missing the last couple
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of years (or at least the good ones), this post is really, really good and will probably
help a lot of people in the community start building extensions. Keep up the good
work Robert.
January 28th, 2009
Michael Kaply
locale and skin are chrome
They should be under the chrome directory.
January 28th, 2009
seth bindernagel
Hey Rey, Nice post. I wrote this a long time ago:
http://blog.mozilla.com/seth/2008/04/07/the-story-of-a-non-hacker-
who-wrote-a-firefox-add-on/
Hope it provides some good stuff to add to your post.
Also, I know guys like Ted M (irc nick luser) developed some pretty handy tools to
get started. I think you can find that stuff here:
https://addons.mozilla.org/en-US/firefox/addon/7434
January 28th, 2009
Paul
Wow ! Nice post
I suggest you to push it on MDC.
January 29th, 2009
Robert Nyman
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Thanks everyone, glad that you like it!
Michael,
That’s a good point, the chrome directory is probably more suitable (just not
mandatory).
January 30th, 2009
Robert Nyman
Paul,
I’d love to have it on the MDC. Rey, what do you think?
January 30th, 2009
Austin King
Another really handy resource by Ted is
http://ted.mielczarek.org/code/mozilla/extensionwiz/
Which will generate all of these files for you into a basic extension that you can
edit and run with.
I just found the FUEL library
https://developer.mozilla.org/en/FUEL
If you are targeting Firefox 3, it’s worth a look. It provides web development
abstractions on top of XUL.
January 30th, 2009
rbango Author
@Robert: I’ll run it by the team and see what we can do.
February 2nd, 2009
robcee
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seconding Paul. This should definitely be on MDC. Great tutorial, Robert. Thanks
for reposting this, Rey!
February 3rd, 2009
Anji
Thanks a ton Robert Nyman
Very useful post
February 5th, 2009
Aditi Mittal
Very nice and informative article
thanx
February 6th, 2009
Robert Nyman
robcee, Anji, Aditi,
Thank you very much!
February 9th, 2009
Lubes
Profile manager on Linux:
1.quit firefox
2.open terminal
3.write command: firefox -ProfileManager
February 26th, 2009
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Jason Thompson
Is there a video on how to do this info overload
March 10th, 2009
Sandeep
hi,
i want to submit a firefox search plugin . pl can you let me know how i can do it.
March 11th, 2009
Mike Camino
Great tutorial, many thanks to you, Robert!
But you said nothing about how to run and debug add-on until it packed and
installed.
I’m very new to mozilla development and it is unclear for me.
March 15th, 2009
Lee
Is there a website where we can request add-ons, like a wish list to add to?
March 22nd, 2009
John Doe
Whoa! Thank you! You’ve saved my day! This is hands down the best article ever
when it comes to add-on development. Thanks!
May 21st, 2009
lj saminathan
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Hai,
Really nice topic . thank you very much . i can understand my project’s extension
very well with your example
July 8th, 2009
Stefan Krüger
Hey Guys,
thanks for the posting and writing off this tutorila.
it was the first helpfull i found^^
meanwhile i have found https://developer.mozilla.org
/en/Building_an_Extension#Test
ther is a realy nice info for beginners:
[quote]…
If you get a message that the install.rdf is malformed, it is helpful to load it into
firefox using the File->Open File command and it will report xml errors to you. In
my case, I had a space before the “<?xml"…
[/quote]
July 17th, 2009
Naveen Mandadi
Very good explanation in understanding of developing an add-on in firefox. This
will be a very good startup for me. I faced a problem while installing the add-on as
per the content of the files in your explanation. xmlns and xmlns:em values in
install.rdf should have RDF in smaller case. This was correct in your download xpi
file.
August 16th, 2009
Naveen Mandadi
How do i know what makes this add-on incompatible to version 3.5.2?
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August 16th, 2009
Naveen Mandadi
Got yah. Incompatibility is because of the maxVersion in the install.rdf
August 16th, 2009
Mike Berthram
Hi, I’m always getting “Empty string in getElementById()” warnings when i open
the options window. How can I solve this?
September 6th, 2009
Mike Berthram
Nvm, i got it. You just have to set an the id attribute for the prefwindow in the
options.xul
September 6th, 2009
pd
wordpress’ ridiculous narrow design really sucks for articles like this. How about a
widescreen version please?
September 13th, 2009
güzel sözler
Whoa! Thank you! You’ve saved my day! This is hands down the best article ever
when it comes to add-on development. Thanks!
September 18th, 2009
kral oyunlar
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Creating extensions for FF and if those are very useful for the users, will make you
popular in this industry.
I wish i could be able to develop even for myself. Thanks for explaining briefly how
to develop.
September 26th, 2009
Oyunlar
Thanx mate! its a good article. but im getting the same problem as Mike Berthram.
Empty string in getElementById()” warnings. can somebody help?
October 2nd, 2009
indir
Whoa! Thank you! You’ve saved my day! This is hands down the best article ever
when it comes to add-on development. Thanks!
October 4th, 2009
Dizi izle
Thanx mate! its a good article. but im getting the same problem as Mike Berthram.
Empty string in getElementById()” warnings. can somebody help?
October 18th, 2009
dizi izle
Hi, i am always getting empty string in getElementById() warning when i open the
options window, how can i solve this ??
October 25th, 2009
promosyon
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Whoa! Thank you! You’ve saved my day! This is hands down the best article ever
when it comes to add-on development. Thanks!
October 28th, 2009
Andreas Eibach
Thank you very much for your work!
Very appreciated, and well-written, too. Keep it up.
November 9th, 2009
blogindir
You’ve saved my day! This is hands down the best article ever when it comes to
add-on development. Thanks!
November 10th, 2009
Andreas Eibach
To user N. Mandadi from Aug 16, 09:
I agree that there might be some beta testers too who use this so it would not hurt
using future version number 3.8 in the downloadable package (I’m testing the 3.7
alphas, so I had to “fix” this too)
To user pd from Sep 13, 09:
I can’t agree more. It’s really a nightmare to scroll back and forth thru the code
samples.
November 10th, 2009
müzik dinle
Nvm, i got it. You just have to set an the id attribute for the prefwindow in the
options.xul
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November 27th, 2009
rafael
Awesome Tutorial!
December 15th, 2009
Gopinath
Hi, I worked it out. It is excellent. It isn’t working with Firefox 3.5. What should i
do to make it.. Thanks in advance
December 22nd, 2009
Shreyas Agasthya
Undoubtedly, this is one of the best articles I came across while I was looking at
my whys, hows, and whats for my plugin development. I have read the complete
article and had hands-on as well. Having done that, I still could not get three
questions answered.
1. Syntax and meaning of chrome.manifest.
2. What does it mean to write chrome://browser/…?
3. Why do we leave /chrome/content when we reference any file underneath this?
Can somebody help me here, please?
December 24th, 2009
GeoNOregon
Very good article. I have no intention nor desire to develop add-ons, and I still
acquired a great understanding of what makes Firefox, Firefox.
Why did I read it? I am trying to find out where add-on’s that have a data file, such
as form entry add-on’s, etc, store the data file. I have been searching for the
answer for a day or two, this time around, (I’ve given up on several previous
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occasions.)
If this was common knowledge, backing up and migrating/upgrading would be a
much simpler process.
Why is it the simplest information about computers and software is often the most
difficult to find?
Later,
GeoD
January 3rd, 2010
Shreyas Agasthya
Hello,
Can someone help me with the above questions?
Regards,
Shreyas
January 5th, 2010
Sandy
hey thanx a lot. this was very useful for my final year project
January 23rd, 2010
Sandy
hello everyone,
I am trying to develop an add-on to firefox for speech recognition which will
enable the user to perform a search through an existing search engine.
so as mentioned in the article i will develop the extension using XUL and java
script.
but what about the speech recognition part..for example lets take feature
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extraction..how do i implement those? should i implement those using another
language and have a pointer to that in java script???
Please some one pay attention to this issue. its for my final year project
Regards,
Sandy
January 23rd, 2010
Firstman
I’m looking for this answer too. Thank you for Q&A
January 26th, 2010
lehenryjr
This is a very good article. I’ve never written a plug-in for Firefox, but I want to
and this gave me a good start. Thanks.
I appreciate it…
————-
Later,
LEHenryJr.
January 26th, 2010
Anup
Really good. Lots of informative stuffs
January 27th, 2010
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