According to the Mozilla Foundation,
Firefox 2 Beta 1 is a developer preview release of our next generation Firefox browser and it is being made available for testing purposes only. Firefox 2 Beta 1 is intended for web application developers and our testing community. Current users of Mozilla Firefox 1.x should not use Firefox 2 Beta 1 and expect all of their extensions and plugins to work properly.
These Release Notes cover what’s new, download and installation instructions, known issues and frequently asked questions for the Firefox 2 Beta 1 release. Please read these notes and the bug filing instructions before reporting any bugs to Bugzilla.
This is one release that I can patiently wait to get my hands dirty upon! I am quite happy with the current version of 1.5.0.4 and am in no hurry to try out the ‘maybe-unstable’ beta! I’d prefer leaving that out to Beta-testers and over-enthusiastic colleagues of mine 
Some new features include a close button on each tab – a feature which I already use courtesy of a nift extension. Another feature is the ‘undo close tab’ (I use that also!) but the most notable one maybe the built-in spell-checker. It would attempt to identify mistakes & spelling errors in text boxes and would underline them! Remains to be seen how useful or how irritating it might be! If anyone’s interested, screenshots of the Beta Release are available here
My Wierd Wired World 