Mozilla recently released the fifth Firefox 3 beta, and I sincerely doubt that it’ll be the last one. Brave souls can try it out, of course, but if you’re a Gmail user I advice against it:
GMail (new version) conversation labels appear on their own row in the message list, and names don’t show in the contacts manager
One of several known issues. There’s bound to be more bugs.
Still, people I’ve talked to who have been running the previous beta tells me it is actually quite stable and performs well. Hopefully we’ll see a release on schedule, which would be June.