Is Firefox on Mac Unusable? - O'Reilly ONLamp Blog: "What are your experiences with FF on Mac? Crashes, hangs? Or is your world just peachy?"
I haven't had any problems at all. Firefox on my macbook pro just works, though their are infrequent crashes. They are rare enough that I am not irritated by them. And when you restart the browser you can just pick up from where you left off, thanks to session restore.
One thing I've learnt is not to play with the alpha/beta releases of Firefox, as they do things to your profile. Uninstalling the alpha/beta milestones, may remove the binary but the damage to your profile is very hard to recover from. Perhaps thats the problem above?