Firefox 3 seems to have stabilized, and is ready for the masses to test it out. The final version should be out by June according to this news report from Reuters:

“The program’s creators told Reuters on Thursday that the privately-held company’s trial version of Firefox 3 browser is ready for the masses to use after months of development.
Engineers at Mozilla are still putting the finishing touches on the software and hope to release the final version of Firefox 3 by the end of June, Schroepfer said.”

Link: Mozilla says Firefox 3 ready for prime-time | Reuters