The Bookmarks Toolbar has been replaced with a Bookmarks Button by default (you can switch it back if you'd like).The stop and reload buttons have been merged into a single button on Windows, Mac OS X, and Linux.You can search for and switch to already open tabs in the Smart Location Bar.Tabs are now on top by default on Windows, Mac OS X, and Linux.Firefox button has a new look for Windows Vista and Windows 7 users.Native support for the HD HTML5 WebM video format, hardware accelerated where available.WebGL is enabled on all platforms that have a capable graphics card with updated drivers.Direct2D Hardware Acceleration is now on by default for Windows 7 users.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).Firefox Sync is included by default, allowing you to securely synchronize between multiple computers and mobile devices.Support for the Do Not Track ('DNT') header that allows users to opt-out of behavioural advertising.Uses JägerMonkey, a new, faster JavaScript engine that is up to six times faster than Firefox 3.6.Firefox 4 is available in over 80 languages.Please read below for more detailed information about what's new in this version of the release, as well as the known issues. 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.

Firefox 4 is based on the Gecko 2.0 Web platform.

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