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Firefox 4 Release Notes

v.4.0.1, released April 28th, 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 4.0.1

Firefox 4.0.1 fixes the following issues found in previous versions of Firefox 4:

Please see the complete list of changes in this version. You may also be interested in the Firefox 4 release notes for a list of changes in the previous version.

System Requirements

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


Mozilla provides Firefox 3.6 for Windows, Linux, and Mac OS X in a variety of languages. You can get the latest version of Firefox 3.6 here. 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.