Setting up an extension development environment

This article gives suggestions on how to set up your Mozilla
application for extension development. Unless otherwise specified, these
suggestions apply to both Firefox and Thunderbird as well as SeaMonkey
version 2.0 and above.

Overview

Create a development user profile
to run your development firefox session; with special development preferences
in

about:config

.

Install some Firefox development extensions
to your dev profile.

Edit files in the extensions folder of your profile and restart the application with the dev profile.

Development profile

To avoid performance degradation from development-related prefs and
extensions, and to avoid losing your personal data, you can use a
separate profile for development work.

You can run two instances of Thunderbird or Firefox at the same time
by using separate profiles and starting the application with the

-no-remote

parameter. For example, the following command will start Firefox with a
profile called "dev" whether an instance of Firefox is already running
or not.

On Ubuntu (and many other Linux distributions):

/usr/bin/firefox -no-remote-P dev

On some other distributions of Linux/Unix:

/usr/local/bin/firefox -no-remote-P dev

On Mac OS Snow Leopard (10.6) and newer:

/Applications/Firefox.app/Contents/MacOS/firefox-bin -no-remote-P dev &

On Mac OS Leopard (10.5) and older, you must request the 32-bit portion of the Universal Binary (https://bugzilla.mozilla.org/show_bug.cgi?id=622970
):

arch -arch i386 /Applications/Firefox.app/Contents/MacOS/firefox-bin -no-remote-P dev &

On Windows:

Start -> Run "%ProgramFiles%/Mozilla Firefox/firefox.exe" -no-remote-P dev

On Windows 64 bit:

Start -> Run "%ProgramFiles(x86)%/Mozilla Firefox/firefox.exe" -no-remote-P dev

To start Thunderbird or SeaMonkey instead of Firefox, substitute
"thunderbird" or "seamonkey" for "firefox" in the examples above.

If the profile specified does not exist (or if no profile is
specified), Thunderbird or Firefox will display the Profile Manager. To
run with the default profile, specify "default" as the profile (or omit
the "-P" switch).

(There is a thread in the Mozillazine forums
that explains how to use both stable and development versions of Firefox to check extension compatibility. See Installing Firefox 3 or Minefield while keeping Firefox 2
.)

To make the numerous restarts easier, you can make one for reading
documentation / for regular use, and the other one for extensions'
testing, and run them at the same time using the

no-remote

command switch as described at http://kb.mozillazine.org/Command_line_arguments
.
The first one often needs lots of restarts, and you can launch both of
them at the same time, like these two shortcuts on Windows:

...firefox.exe -no-remote-p "profile1"

...firefox.exe -no-remote-p "profile2"

These are Windows-specific lines. If you find the same instructions for Linux / Mac OS / etc please place them here too.
replace "..." with some real path. It is simple - just two
shortcuts on the desktop - and lets you keep one of the profiles always
open.

Development command flags

As of Gecko 2 (Firefox 4), JavaScript files are cached ("fastload"). The

-purgecaches

command-line flag disables this behavior. Alternatively, you can set the MOZ_PURGE_CACHES environment variable. See this bug
for more information.

Development preferences

There is a set of development preferences that, when enabled, allows
you to view more information about application activity, thus making
debugging easier. However, these preferences can degrade performance,
so you may want to use a separate development profile when you enable
these preferences. For more information about Mozilla preferences, refer
to the mozillaZine article on "about:config
".

Not all preferences are defined by default, and are therefore not listed in

about:config

by default. You will have to create new (boolean) entries for them.

javascript.options.showInConsole
= true
. Logs errors in chrome files to the Error Console
.

nglayout.debug.disable_xul_cache
= true
. Disables the XUL cache so that changes to windows and dialogs do not require a restart. This assumes you're using directories rather than JARs
. Changes to XUL overlays will still require reloading of the document overlaid.

browser.dom.window.dump.enabled
= true
. Enables the use of the dump()
statement to print to the standard console. See

window.dump

for more info. You can use

nsIConsoleService

instead of

dump()

from a privileged script.

javascript.options.strict
= true
.
Enables strict JavaScript warnings in the Error Console. Note that
since many people have this setting turned off when developing, you will
see lots of warnings for problems with their code in addition to
warnings for your own extension. You can filter those with Console2

.

devtools.chrome.enabled = true.
This enables to run JavaScript code snippets in the chrome context of
the Scratchpad from the Tools menu. Don't forget to switch from content
to chrome as context.

extensions.logging.enabled
= true
. This will send more detailed information about installation and update problems to the Error Console
.
(Note that the extension manager automatically restarts the application
at startup sometimes, which may mean you won't have time to see the
messages logged before the automatic restart happens. To see them,
prevent the automatic restart by setting the environment NO_EM_RESTART
to 1 before starting the application.)

nglayout.debug.disable_xul_fastload = true
. For Gecko 2.0+ (Firefox 4.0+). See this bug
for more information. Although the bug has been closed, it is believed that this pref is still relevant.

You might also want to set dom.report_all_js_exceptions = true
. See Exception logging in JavaScript
for details.

Gecko 2.0 note

(Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1)

The Error Console is disabled by default starting in Firefox 4. You can re-enable it by changing the

devtools.errorconsole.enabled

preference to

true

and restarting the browser. With this,

javascript.options.showInConsole

is also set to

true

by default.

Accessing Firefox development preferences

To change preference settings in Firefox or SeaMonkey, type

about:config

in the Location Bar. Alternatively, use the Extension Developer's Extension
, which provides a menu interface for Firefox settings.

Accessing Thunderbird development preferences

To change preference settings in Thunderbird, open the "Preferences"
(Unix) or "Options" (Windows) interface. On the "Advanced" page, select
the "General" tab then click the "Config Editor" button.

Development extensions

These extensions may help you with your development.

DOM Inspector
, used to inspect and edit the live DOM of any web document or XUL application (Firefox and Thunderbird)

Venkman
, a JavaScript Debugger (Firefox version
, Thunderbird version
)

Extension Developer's Extension
a suite of tools for extension development (Firefox)

Extension Test
an add-on which makes it easier to detect problems which will lead to rejection by addons.mozilla.org

Console²
enhanced JavaScript console (Firefox version
, Thunderbird version
)

Javascript Command
for writing/testing javascript on Firefox windows

Chrome List
navigate and view files in chrome:// (Firefox version
, Thunderbird version
)

Chrome Edit Plus
a user file editor (Firefox and Thunderbird)

Add-on Builder
a web-based application that generates an extension skeleton (Firefox, Thunderbird, and others)

Firebug
a variety of development tools (Firefox)

Pentadactyl
,
a general purpose extension with builtin tools for extension
development, including a command line with chrome JavaScript evaluation
(including property and function argument completion) and the ability to
demand-load external JavaScript and CSS files into window chrome.

Chromebug
combines elements of a JavaScript debugger and DOM (Firefox, "kinda works for Thunderbird")

MozRepl
explore and modify Firefox and other Mozilla apps while they run (Firefox and Thunderbird)

ExecuteJS
an enhanced JavaScript console (Firefox version
, Thunderbird version
)

XPCOMViewer
an XPCOM inspector (Firefox and Thunderbird)

JavaScript shells
to test snippets of JavaScript (Firefox and Thunderbird)

SQLite Manager
to manage the SQLite database (Firefox and Thunderbird)

ViewAbout
enables access to various about: dialogs from the View menu (Firefox version
, Thunderbird version
)

Crash Me Now!
useful for testing debug symbols and the crash reporting system (Firefox and Thunderbird)

Firefox extension proxy file

Extension files are normally installed in the user profile. However,
it is usually easier to place extension files in a temporary location,
which also protects source files from accidental deletion. This section
explains how to create a proxy file that points to an extension that is
installed in a location other than the user profile.

Get the extension ID from the extension's install.rdf file.

Create a file in the "

extensions

" directory under your profile directory with the extension's ID as the file name (for example "

your_profile_directory/extensions/{46D1B3C0-DB7A-4b1a-863A-6EE6F77ECB58}

"). (How to find your profile directory
) Alternatively, rather than using a GUID, create a unique ID using the format "name@yourdomain" (for example

chromebug@mydomain.com

).

The contents of this file should be the path to the directory that contains your install.rdf file. (eg. "

/full/path/to/yourExtension/

". Windows users should use the drive name (CAPS) and backslashes instead of frontslashes (for example "

C:/full/path/to/yourExtension/

" or "

C:/sam/workspace/toolbar/helloWorldtoolbar/

"). Remember to include the closing slash and remove any trailing whitespace.

Important: In
Firefox 3, if you already installed the extension via XPI, you might
need to delete one or all of the extensions.* files in the profile
folder. Backup first, but these files will be regenerated. (I don't know
what the original author had in mind here, but the files

extensions.cache

,

extensions.ini

,

extensions.rdf

,
are
all regenerated by Firefox if deleted. You need to delete them if you
mess with 'components', no harm done. If you do not initially delete
these files the first time you try to load your extension, then the
extension will silently not load. Firefox will disconnect from its
parent process when it regens these files, so you may have to exit once
and restart if you use the OS console.)

Important: Note that
the use of proxy files requires that the extension's chrome.manifest
defines its chrome urls using traditional directories, rather than a
JARed structure. See below.

Place the file in the extensions folder of your profile and restart the application.

Using directories rather than JARs

Regardless of whether you choose to eventually package your
extension's chrome in a JAR or in directories, developing in directories
is simpler. If you choose a JARed structure for releasing, you can
still develop with a directory structure by editing your
chrome.manifest. For example, rather than having

content	myExtension	jar:chrome/myExtension.jar!/content/

use

content	myExtension	chrome/content/