You are viewing information archived from on 2014-02-12.

Firefox 4 Beta

Beta version 1: released July 6th, 2010

These are the release notes for the first version of the Firefox 4 Beta, based on the Gecko 2.0 Web platform. Please read below for more detailed information about what's new in this version of the beta release, as well as the known issues.

We rely on your feedback to help shape our products, so we've bundled a Feedback Add-On with the Firefox 4 Beta that provides an easy to use Feedback button.

If you want to test Add-ons with Firefox 4 Beta, you will need to install the Add-On Compatibility Reporter first.

About this release

Firefox 4 Beta (version 1) considered to be stable and safe to use for daily web browsing, though the features and content may change before the final product release. At this time many Add-ons may not yet have been tested by their authors to ensure that they are compatible with this release. If you wish to help test Add-on compatibility, please install the Add-on Compatibility Reporter - your favorite Add-on author will appreciate it!

Check out what’s new, the known issues and frequently asked questions about the latest version of Firefox. Please tell us what you think using the feedback tools provided as part of the beta. We appreciate your input!

What’s New in Firefox 4 Beta (revision 1)

  • Tabs are now on top by default on Windows only - OSX and Linux will be changing when the theme has been modified to support the change.
  • On Windows Vista and Windows 7 the menu bar has been replaced with the Firefox button.
  • You can search for and switch to already open tabs in the Smart Location Bar
  • 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.
  • The stop and reload buttons have been merged into a single button on Windows, Mac 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 when there is a crash in the Adobe Flash, Apple Quicktime or Microsoft Silverlight plugins.
  • CSS Transitions are partially supported.
  • Full WebGL support is included but disabled by default at this time.
  • Core Animation rendering model for plugins on Mac OS X. Plugins which also support this rendering model can now draw faster and more efficiently.
  • Native support for the HD HTML5 WebM video format.
  • An experimental Direct2D rendering backend is available on Windows, turned off by default.
  • Web developers can use Websockets for a low complexity, low latency, bidirectional communications API.
  • 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.

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 will provide Firefox 4 Beta for Windows, Linux, and Mac OS X in a variety of languages, though this first version is only available in English. Eventually you will be able to get the latest version of the Firefox 4 Beta in many 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 Beta will not 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 Beta with the Add/Removes Programs utility on Windows, by moving the Firefox application to the Trash on OS X, or by deleting the containing folder on Linux.

By default, removing Firefox 4 Beta 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 Beta will continue to use the bookmarks, web browsing history, add-ons, and other data from this profile folder.

Add-ons and Themes

Add-ons installed with previous versions of Firefox may not yet have been updated by their authors to work with Firefox 4 Beta. 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 Beta which will be resolved in future versions:

All Systems
  • The Bookmark Toolbar is turned off by default even on existing profiles (see bug 574514)
  • If you try to start Firefox using a locked profile, it will crash (see bug 573369)
  • After using the Customize Toolbars feature, the Bookmarks Toolbar context menu will not work until you restart the browser (see bug 557225)
  • Rendering of the popular Tuenti website is broken, though already fixed for the next beta (see bug 573969)
Microsoft Windows
  • Some graphics drivers may not work properly with full screen Ogg Theora HTML video acceleration (see bug 564391)
  • When using a Persona, the maximize, minimize and close window buttons will not appear, though they will still work (see bug 574833)
  • When using the Windows "Classic" theme the toolbars and tab strip may appear gray for several seconds as Firefox starts up (see bug 574638)
Mac OS X
  • Although you can customize Firefox to show tabs on top for OSX, it really doesn't look good. We suggest you wait.
  • When plugin content is loaded in a separate process, a second icon appears in the OSX Dock (see bug 557225).
Linux and Unix


  • 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 favourite 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 Beta?

    Lots of people. See Help->About Mozilla Firefox, Credits for a list of some of the people who have contributed to Firefox 4 Beta.

  4. Where’s the Firefox 4 Beta source code?

    A tarball of the Firefox 4 Beta 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 Beta 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.