I've been using the new Firefox beta for a whole day now and not one crash or lockup. Of course I don't have my extensions installed now either, which I suspect is usually the culprit for most problems, although the occasional mishandled piece of html seemed to be a problem every once in a while.
I like the more mac-like skin (especially in forms) in the mac version. It seesm pretty zippy, too.
The Mozilla Blog