Firefox 4.0 Beta 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.