No problems here with Firefox 5 & now Firefox 6.
Had installed the 2nd day I got the Mac mini, about 3 weeks ago; haven't had any crashes.
Did have "crashes" at some websites because I don't have javascript on, normally, & only turn it on when necessary after which I turn javascript off again. Sometimes, a website would send the javascript then expects a respond which will never happen; & some websites get the wait forever.
In either case, the website's configuration will lock up the browser so I would have to kill the browser myself (this is a crash or malware prevention?) Sometimes, just disabling javascript will fix the situation.
I've found that the Mac apps are usually multi-threaded so that the "preferences" can sit on a separate window while browsing; much easier to turn on/off javascript unlike windows which is still locked into single threaded apps, even Firefox (no official 64-bit version there).