As far as Firefox releases go, I've been so unimpressed with the latest 3.5.1 version I've done what I've never done before and reverted to an earlier version. For me it's been more agitating than the memory leak issue from back in the 2.x days because instead of just steadily getting slower this release is close to being unusable.
First of all, it crashes. A lot. On a good afternoon it will crash around 5-10 times an hour. It can restore all my tabs after a crash without any problems other than the mild irriration of re-entering in my login credentials for each page that were closed, but it's still obviously a pain. The second issue is tabs will sometimes ignore mouse input, meaning I have to use keyboard shortcuts to reach certain tabs.
I'm not sure whether it's something to do with my extensions, my Core Duo MacBook Pro platform or a combination of the two, but I've moved back to the 3.0 series with 3.0.12 and everything has been working beautifully since.
I rely on Firefox because of all my privacy and security extensions that no other browser even comes close to matching (perhaps that will change), but ironically such plugins might also be it's archilies heel when it comes to reliable performance. Bummer.