So far the latest beta version is working well in early testing
Firefox 4 Beta 12 released
Firefox 4 Beta 12 – Release Notes
QUOTE: For those of you who would like to contribute to the future of Firefox, while not quite ready for final release, Firefox 4 Beta 12 is considered “stable and safe to use for daily browsing”. There are still some known issues and the Mozilla people do warn that if you are a add-on user there may be some issues with your add-ons, but with the Add-on Compatability Reporter you can assist the add-on developers as well.