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.


  • 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 parameters -no-remote and -P ProfileName. For example, the following command will start Firefox with a profile called "dev" whether an instance of Firefox is already running or not. (If there is no "dev" user yet, you'll get the profile selection screen instead, where you can create one.)

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/ -no-remote -P dev &

On Mac OS Leopard (10.5) and older, you must request the 32-bit portion of the Universal Binary (

arch -arch i386 /Applications/ -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.

Note that you can run Firefox using your regular profile while developing.

Parameter -P ProfileName doesn't imply -no-remote, therefore use them together. Otherwise, if you already run a Firefox instance without -no-remote, and you attempt to start another instance with -P ProfileName but again without -no-remote, that second invocation would ignore its -P ProfileName parameter, but instead it would open a new blank window for the already running instance (sharing its profile, sessions etc.).

(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.)

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.

Accessing Firefox development preferences

To change preference settings in Firefox or SeaMonkey, type about:config in the Location Bar. You can also use the Extension Developer's Extension, which provides a menu interface for Firefox settings. Alternatively, install the Developer Profile to set the preferences listed below and skip the rest of this section.

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.

Not all preferences are defined by default, and are therefore not listed by default. You will have to create new (boolean) entries for them. For more information about Mozilla preferences, refer to the mozillaZine article on "about:config". (Tip: Download addon DevPrefs, it will automatically handle this)

  • 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.
  • = 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 browser as context.
  • devtools.debugger.remote-enabled = true. This adds a "Browser Debugger" entry to the "Web Developer" submenu of the "Tools" menu.  The Browser Debugger can be used to debug the JavaScript code of extensions. The preference must also be set to true for the Browser Debugger to be enabled.
  • 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.
  • devtools.errorconsole.deprecation_warnings = true. Detect deprecated code use.

Warning: Never set nglayout.debug.disable_xul_fastload to true in a production environment; it exists solely to aid in debugging. In particular, add-ons should never change this preference.

Note:The Error Console is disabled by default starting in Gecko 2.0. 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.

Tip: Download the addon DevPrefs from AMO to automatically configure the preferences.

Development extensions

These extensions may help you with your development.

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.

  1. Get the extension ID from the extension's install.rdf file.
  2. 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 - then the proxy filename will be same as that ID, with no curly brackets {}.
  3. The contents of this file should be the path to the directory that contains your install.rdf file, for example /full/path/to/yourExtension/ on Mac and Linux, and C:\full\path\to\yourExtension\ on Windows. Remember to include the closing slash and remove any trailing whitespace.

    • Note: If you already installed the extension via XPI, you should uninstall it first before creating the pointer file.
    • Also 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.
  4. 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/


content	myExtension	chrome/content/

Preventing the first launch extension selector

(Firefox 8.0 / Thunderbird 8.0 / SeaMonkey 2.5)

Starting in Firefox 8, on the first launch of a new version of Firefox, it presents user interface letting users select which third party add-ons to keep. This lets them weed out add-ons that were installed without their knowledge, or that are no longer needed.

However, this interface can be disruptive when debugging add-ons. You can avoid this by setting the preference extensions.autoDisableScopes to 14.

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