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Firefox 4 Release Notes

v.4.0, released March 22nd, 2011 Check out what’s new, the known issues and frequently asked questions about the latest version of Firefox. As always, you’re encouraged to tell us what you think, or file a bug in Bugzilla.

What’s New in Firefox 4

Firefox 4 is based on the Gecko 2.0 Web platform. This release features JavaScript execution speeds up to six times faster than the previous version, new capabilities for Web Developers and Add-on Developers such as hardware accelerated graphics and HTML5 technologies, and a completely revised user interface. Please read below for more detailed information about what's new in this version of the release, as well as the known issues.

  • Firefox 4 is available in over 80 languages
  • Uses JägerMonkey, a new, faster JavaScript engine that is up to six times faster than Firefox 3.6
  • Support for the Do Not Track ("DNT") header that allows users to opt-out of behavioural advertising
  • Firefox Sync is included by default, allowing you to securely synchronize between multiple computers and mobile devices
  • Certain graphics rendering operations are now hardware-accelerated using Direct3D 9 on Windows XP, Direct3D 10 on Windows Vista and 7, and OpenGL on Mac OS (OpenGL on Linux will be supported in the future)
  • Direct2D Hardware Acceleration is now on by default for Windows 7 users
  • WebGL is enabled on all platforms that have a capable graphics card with updated drivers
  • Native support for the HD HTML5 WebM video format, hardware accelerated where available
  • Firefox button has a new look for Windows Vista and Windows 7 users
  • Tabs are now on top by default on Windows, Mac OS X, and Linux
  • You can search for and switch to already open tabs in the Smart Location Bar
  • The stop and reload buttons have been merged into a single button on Windows, Mac OS X, and Linux
  • The Bookmarks Toolbar has been replaced with a Bookmarks Button by default (you can switch it back if you'd like)
  • Crash protection for Windows, Linux, and Mac OS X when there is a crash in the Adobe Flash, Apple Quicktime or Microsoft Silverlight plugins
  • You can turn any tab into an "App Tab" by right-clicking on it and selecting "Make into App Tab" from the context menu
  • The default homepage design has been refreshed
  • Overhaul of the bookmarks and history code, enabling faster bookmarking and startup performance
  • Per-compartment garbage collection is now enabled, reducing work done during complex animations
  • Additional polish for the Firefox Add-ons Manager
  • Improved web typography using OpenType with support for ligatures, kerning and font variants
  • Web developers can animate content using CSS Transitions
  • Responsiveness and scrolling improvements from the new retained layers layout system
  • HTML5 Forms API makes web based forms easier to implement and validate
  • Support for the new proposed Audio Data API
  • Support for HSTS security protocol allowing sites to insist that they only be loaded over SSL
  • A new feature called Panorama gives users a visual overview of all open tabs, allowing them to be sorted and grouped
  • An experimental API is included to provide more efficient Javascript animations
  • Firefox now supports the HTML5 video "buffered" property
  • Changes to how XPCOM components are registered in order to help startup time and process separation
  • New Addons Manager and extension management API (UI will be changed before final release)
  • Significant API improvements are available for JS-ctypes, a foreign function interface for extensions
  • CSS Transitions are partially supported
  • Core Animation rendering model for plugins on Mac OS X. Plugins which also support this rendering model can now draw faster and more efficiently
  • Web developers can update the URL field without reloading the page using HTML History APIs
  • More responsive page rendering using lazy frame construction
  • Link history lookup is done asynchronously to provide better responsiveness during pageload
  • CSS :visited selectors have been changed to block websites from being able to check a user's browsing history
  • New HTML5 parser
  • Support for more HTML5 form controls
  • Web authors can now get touch events from Firefox users on Windows 7 machines
  • A new way of representing values in JavaScript that allows Firefox to execute heavy, numeric code (used for things like graphics and animations) more efficiently

Developers can find out about all the changes and new features at the Mozilla Developer Center.

System Requirements

Before installing, make sure your computer meets the system requirements.


Mozilla provides Firefox 4 for Windows, Linux, and Mac OS X in a variety of languages. For builds for other systems and languages not provided by Mozilla, see the Contributed Builds section at the end of this document.


Please note that installing Firefox 4 will overwrite your existing installation of Firefox. You won’t lose any of your bookmarks or browsing history, but some of your extensions and other add-ons might not work until updates for them are made available.


You can remove Firefox 4 with the Add/Remove Programs utility on Windows, by moving the Firefox application to the Trash on Mac OS X, or by deleting the containing folder on Linux.

By default, removing Firefox 4 won’t remove your bookmarks, web browsing history, extensions or other add-ons. This data is stored in your profile folder, which can be found by going to the Help menu and selecting Troubleshooting Information.... The button next to the Profile Directory line in Application Basics will open your profile directory in your system's file explorer.

Please note that if you keep your profile, any version of Firefox that you install after removing Firefox 4 will continue to use the bookmarks, web browsing history, add-ons, and other data from this profile folder.

Add-ons and Themes

You can get Add-ons and Themes by opening the Add-ons Manager, or by visiting the Add-ons Web site. Add-ons installed with previous versions of Firefox may not yet have been updated by their authors to work with this Firefox 4. If you wish to help test Add-ons, please install the Add-on Compatibility Reporter - your favorite Add-on author will appreciate it!

Known Issues

This list covers some of the known problems with Firefox 4 which will be resolved in future versions:

All Systems
  • Users will experience many incompatibilities with bookmarks and history if they downgrade to any Firefox 4 beta before beta 8. Users can still downgrade back to Firefox 3.6 (see bug 620723 and bug 614439)
  • Users using Firefox Sync with their mobile device will need to update to the latest Firefox 4 for mobile release
  • For some users, scrolling in the main GMail window will be slower than usual (see bug 579260)
  • If you try to start Firefox using a locked profile, it will crash (see bug 573369)
Microsoft Windows
  • Some users of Adobe Reader X have experienced instability when viewing PDF documents in the browser. Uninstalling and reinstalling Adobe Reader X has been determined to resolve the issue (see bug 640901)
  • If you customize your toolbars to include the Location Bar or Search Bar in the Menu Bar, and then toggle the state of the Menu Bar between shown and hidden, Firefox will crash (see bug 640652)
Mac OS X
  • This version of the beta will not work on Macintosh hardware with Power PC CPUs (see bug 587799)
  • Firefox on Mac OS X running in 64-bit mode does not recognize that Java supports certain MIME types. Running Firefox in 32-bit mode has been found to help (see bug 638171)
Linux and Unix
  • The video control buttons may not work when viewing QuickTime videos with libtotem (see bug 625036)
  • Users compiling from source might need a newer gcc and libstdc++ as the build requirements have changed (see bug 578880)


  • Poorly designed or incompatible extensions can cause problems with your browser, including make it crash, slow down page display, etc. If you encounter strange problems relating to parts of the browser no longer working, the browser not starting, windows with strange or distorted appearance, degraded performance, etc, you may be suffering from Extension or Theme trouble. Restart the browser in Safe Mode. On Windows, start using the "Safe Mode" shortcut created in your Start menu or by running firefox.exe -safe-mode. On Linux, start with ./firefox -safe-mode and on Mac OS X, run:

    cd /Applications/
    ./firefox-bin -safe-mode

    When started in Safe Mode all extensions are disabled and the Default theme is used. Disable the Extension/Theme that is causing trouble and then start normally.
  • If you uninstall an extension that is installed with your user profile (i.e. you installed it from a Web page) and then wish to install it for all user profiles using the -install-global-extension command line flag, you must restart the browser once to cleanse the profile extensions datasource of traces of that extension before installing with the switch. If you do not do this you may end up with a jammed entry in the Extensions list and will be unable to install the extension globally.
  • If you encounter strange problems relating to bookmarks, downloads, window placement, toolbars, history, or other settings, it is recommended that you try creating a new profile and attempting to reproduce the problem before filing bugs. Create a new profile by running Firefox with the -P command line argument, choose the "Manage Profiles" button and then choose "Create Profile...". Migrate your settings files (Bookmarks, Saved Passwords, etc) over one by one, checking each time to see if the problems resurface. If you do find a particular profile data file is causing a problem, file a bug and attach the file.

Frequently Asked Questions

  1. What can I do to help?

    We need help from developers and the testing community to provide as much feedback as possible to make Firefox even better. Please pick your favorite way to give us your feedback at any time!

  2. Where can I get extensions and themes (add-ons)?

    Extensions and Themes can be downloaded from Firefox Add-ons.

  3. Who makes Firefox 4?

    Lots of people. See Help->About Mozilla Firefox, and click the "global community" for a list of some of the people who have contributed to Firefox 4.

  4. Where’s the Firefox 4 source code?

    A tarball of the Firefox 4 source code is available for download. The latest development code can be obtained through Mercurial. Firefox-specific source is in mozilla-central's "browser", "toolkit", and "chrome" directories. Please follow the build instructions.

  5. Where is the mail client?

    Firefox 4 works with whatever mail client is the default on your system. However, we recommend Mozilla Thunderbird, our next-generation email client and the perfect complement to Firefox.

Contributed Builds

These are unofficial builds and may be configured differently than the official Mozilla builds. They may also be optimized and/or tested for specific platforms. You can browse through the available contributed builds on the FTP site.