I've been updating as the new versions come out, and not had any new problems. I might have a go with 4.0 Beta, though. I used to run three levels of Firefox on the PC in the days when they had code names (I forget, now, what they were). I stopped that when I was playing with Windows 7 Beta and didn't want too much instability.
The old problem remains: about once a day Firefox will crash, tell me how embarrassed it is, ask to send a notification to Mum, and offer to restore my tabs.
I can't blame it entirely for that, because I like to be in control of what my machine does. I don't let it put the config stuff umpty levels down in the bowels of C: drive - I have it in my "Internet" drive (drive I, cos I have a vivid imagination) in a folder called Firefox. The weekly backup routine copies the config file, bookmarks, and the like.
Thunderbird likewise is constrained to put everything into I
Mail where I can see when the .msf files get over-bloated, and delete them.
I've got the "new" version of Thunderbird on the laptop, but I prefer having the "Reply", "Forward" etc buttons up in the main toolbar rather than down in the middle of the page so I've not gone "new" on the main machine.