Firefox 4 includes a laundry list of new or improved technologies, including the Gecko 2.0 engine, Websockets, enhanced HTML5 support (including WebM video support), partial CSS Transitions support, and crash protection (for the Flash, QuickTime and Silverlight plugins).
There's also full support for WebGL (disabled by default), a Windows Direct2D rendering backend (also disabled by default), Mac OS X Core Animation support for plugins, and HTML History APIs.
Responsiveness has been improved through lazy frame construction (which reduces the number of times complex pages are reflowed) and asynchronous link history lookup.
Talking of link history, user privacy has been improved by blocking websites from reading the browsing history.
What else has changed? Please read on.
These include 'tabs on top' (the option of showing tabs Chrome-style above the Awesome bar; currently only in the Windows version), a new Addons Manager (still under development), the merging of the Stop and Reload buttons, and a Bookmarks button in place of the Bookmarks bar (the latter can be restored by the user).
And on Windows 7 and Vista, the new (though unfinished) Firefox button groups commands such as New Window, Save Page, Print, Options, and Exit that were traditionally shown in the menu bar.
Further beta releases are planned at two to three week intervals. "There will be much more to test in future beta releases and not everything that you see in this beta is guaranteed to be in Firefox 4," said Mike Beltzner, director of Firefox at Mozilla Corporation.