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Firefox 3.5

v.3.5, released June 30th, 2009 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, either using this feedback form or by filing a bug in Bugzilla.

What’s New in Firefox 3.5

Firefox 3.5  is based on the Gecko 1.9.1 rendering platform, which has been under development for the past year. Firefox 3.5 offers many changes over the previous version, supporting new web technologies, improving performance and ease of use. Some of the notable features are:

  • Available in more than 70 languages. (Get your local version!)
  • Support for the HTML5 <video> and <audio> elements including native support for Ogg Theora encoded video and Vorbis encoded audio. (Try it here!)
  • Improved tools for controlling your private data, including a Private Browsing Mode.
  • Better web application performance using the new TraceMonkey JavaScript engine.
  • The ability to share your location with websites using Location Aware Browsing. (Try it here!)
  • Support for native JSON, and web worker threads.
  • Improvements to the Gecko layout engine, including speculative parsing for faster content rendering.
  • Support for new web technologies such as: downloadable fonts, CSS media queries, new transformations and properties, JavaScript query selectors, HTML5 local storage and offline application storage, <canvas> text, ICC profiles, and SVG transforms.

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 3.5 for Windows, Linux, and Mac OS X in a variety of languages. You can get the latest version of Firefox 3.5 here. For builds for other systems and languages not provided by, see the Contributed Builds section at the end of this document.


Please note that installing Firefox 3.5 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 reinstall an older version later if you wish to downgrade.


You can remove Firefox 3.5 through the Control Panel in the Start Menu on Windows, by removing the Firefox application on OS X, or by removing the firefox folder on Linux.

Removing Firefox 3.5 won’t remove your bookmarks, web browsing history, extensions or other add-ons. This data is stored in your profile folder, which is located in one of the following locations depending on your operating system:

Windows Vista Users\<UserName>\AppData\Roaming\Mozilla\Firefox
Windows 2000, XP, Server 2003 Documents and Settings\<UserName>\Application Data\Mozilla\Firefox
Mac OS X ~/Library/Application Support/Firefox
Linux and Unix systems ~/.mozilla/firefox

Any version of Firefox that you install after removing Firefox 3.5 will continue to use the data from this profile folder.

Extensions and Themes

Extensions installed under Firefox 3 may be incompatible and/or require updates to work with Firefox 3.5. Please report any issues to the maintainer of the extension. When you install Firefox 3.5 all of your Extensions and Themes will be disabled until Firefox 3.5 determines that either a) they are compatible with the Firefox 3.5 release or b) there are newer versions available that are compatible.

Known Issues

This list covers some of the known problems with Firefox 3.5 which will be fixed in future updates or releases:

All Systems
  • After installing this version, users will not be able to return to a previous beta version of Firefox 3.5 without creating a new profile (see bug 488966)
  • Restoring your bookmarks from a backup can take some time, during which the browser may seem unresponsive (see bug 493731)
  • If a pop-up window is focused when you enter Private Browsing Mode, the Location Bar in the first Private Browsing Mode window will not be editable. Opening a new window fixes the problem (see bug 495495)
  • After using Clear Recent History some SSL sites will not load all images and styles without pressing reload (see bug 480619)
  • When editing Arabic, Hebrew, Hindi or Persian scripts, the characters may appear disconnected. Reloading the page or copy and pasting the content over top of itself will fix the problem (see bug 499538)
  • Firefox 3.5 will only support color profiles as specified by ICC v2, later versions will add support for ICC v4 (see bug 463221)
  • Users viewing images using ICC v2 color profiles on monitors with wide gamut levels may find them to look more saturated (darker) than expected (see bug 497363)
  • Websites using the new downloadable font support that wish to change the color using :hover will also need to add text-rendering : optimizeLegibility for font sizes less than 20 pixels (see bug 495455)
  • Some sites which require Java to Javascript communication may not work properly (see bug 484744)
  • Some sites with Flash can cause problems with the Cookies dialog (see bug 495035)
  • Web pages can no longer automatically install PKCS11 cryptographic tokens. Users are now required to do this manually or install an Add-on that installs them (see instructions and documentation)
Microsoft Windows
  • Pressing enter in the Location Bar will not do anything if you are running AVG SafeSearch v8.0 or lower. Upgrading AVG SafeSearch fixes the problem (see bug 479095)
  • If you select Fax from the Print Dialog, the Windows Fax dialog will appear and then disappear. Faxing still works from other applications (see bug 440486)
Mac OS X
  • Java applications that rely on tracking the mouse position may not function properly (see bug 495278)
  • If the user repeatedly changes the paper size and asks for a Print Preview, Firefox may crash (see bug 495567)
Linux and Unix
  • Zooming out on some sites may result in grey or black lines appearing (see bug 477552)


  • 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 read these notes and the bug filing instructions before reporting any bugs to Bugzilla. You can also give us your feedback through this feedback form.

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

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

  3. Who makes Firefox 3.5?

    Lots of people. See Help->About Mozilla Firefox, Credits for a list of some of the people who have contributed to Firefox 3.5.

  4. Where’s the Firefox 3.5 source code?

    A tarball of the Firefox 3.5 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 3.5 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.