Not that I've noticed, but as with every software upgrade, it's highly advisable to backup your mail before you upgrade.
I looked on my T-bird and can't find the backup function.
BTW, about two weeks ago I accidentally uninstalled T-Bird. Two days ago I reinstalled it and everything was still there. All the mail, contacts, etc.
I was happily surprised yet a bit confused. Why didn't the uninstall remove all that stuff?