• Firefox 4 Beta 2 is available in 24 languages.
  
• Tabs are now on top by default on Windows and OSX – Linux will be
  changing when the theme has been modified to support the change.
  
• You can turn any tab into an “App Tab” by right-clicking on it and selecting “Make into App Tab” from the context menu.
  
• Web developers can animate content using CSS Transitions.
  
• Responsiveness and scrolling improvements from the new retained layers layout system.
  
• JavaScript speed improvements due to engine optimizations.
  
• Changes to how XPCOM components are registered in order to help startup time and process separation.
  
• See the complete changelist from the previous beta.
  
• 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.