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 tool bar rather than down in the middle of the page so I've not gone "new" on the main machine.