If you're a Mac Firefox user, I'd recommend staying away from the recently released 3.0.5 update. I stupidly installed it automatically (via Preferences > Advanced > Update) on my MacBook Pro 2.4GHz running Mac OS 10.5.5 and it's completely FUBAR.
Tabs don't work, neither Command-T nor File > New Tab produce a new tab and instead FF 3.0.5 stares back blankly at me. And now FF305 won't even launch for me - it just beachballs until I force quit it. I've resorted to using Firefox 3.1 beta 2 in the interim, because at least it launches.
I know that FF 3.0.5 is probably working fine for you, but other Mac-heads are also complaining about problems in the Mozilla forums. I'm just sayin'...
If you've got a FireFix that doesn't involve upgrading to 10.5.6 (too close to Macworld Expo to risk it) I'd love to hear about it in the TalkBack.