After numerous rumors and false alarms Firefox 3 Beta 1 has been officially released today. New in Firefox 3 is the implementation of Gecko 1.9, an updated layout engine which among other things should permit on-the-fly website zooming which is already supported in Opera.
Firefox 3 a.k.a. Gran Paradiso is also expected to add new features and fix a number of bugs, for example this version will be the first official release of a Mozilla browser to pass the Acid2 test. It was also reported recently that depending on the operating system you run, the Firefox 3 GUI will be implemented differently and optimized for every major OS release out there.
As with all beta software try this at your own risk, although it shouldn't be a bad idea to give it a shot without replacing your current installation of Firefox 2 (if that is what you currently use). It is to be expected that not all extensions will work fine with Firefox 3.