I really like both Firefox and Thunderbird, and I like using the latest stable versions (plus it makes sense from a security standpoint). Unfortunately, the official Ubuntu software repositories lag behind, so if you stick to them you don't get the latest versions. In my case, that's an extra headache because I also use Firefox and Thunderbird on Windows XP, and I'd like to have the same user interface on all machines. Fortunately, there's a simple solution: the Ubuntuzilla project.
The first step is to add Ubuntuzilla to the list of repositories used by the Synaptic package manager. The Ubuntuzilla wiki gives good instructions, although they seem to omit what I would consider the most intuitive approach for GUI-oriented users: start the package manager (giving an administrator password); click Settings | Repositories; go to the Other Software tab; click Add... and paste in the command shown on the wiki page ("deb ... main"). Then click the Reload button. (Ignore any complaint about not finding the public key for the new repository.) You can now install the firefox-mozilla-build and thunderbird-mozilla-build packages to update them to the latest versions, and also do future updates through Synaptic the same as with other packages.
Before updating them, though, it's recommended that you back up your existing configuration information, mailboxes etc. They'll most likely be saved in folders named .mozilla and .mozilla-thunderbird under your home folder. If you're using the default Nautilus file manager, you can see hidden folders (those starting with ".") by hitting ctrl-h. Just copy those two folders anywhere safe (I use the desktop) before upgrading.
Today's update took Firefox from 3.5 to 3.6.2 and Thunderbird from 2.0.something to 3.0.3. Firefox found and used the old settings with no effort, but Thunderbird did not. (The same upgrade on Windows did automatically convert all the old settings, archives etc.) The problem with Thunderbird is that version 2 stored its stuff in .mozilla-thunderbird but version 3 stores it in .thunderbird. So the fix is trivial. The first step is to create a .thunderbird folder under the home folder. If you've already started Thunderbird and bailed out when it asked you to create new accounts, just find the .thunderbird folder and delete its contents (but leave the folder itself). Then all that remains is to copy the contents of .mozilla-thunderbird into .thunderbird, and Tbird 3 aborbs it all when next it is started. I'll hang onto the .mozilla-thunderbird folder a bit just to be on the safe side, but I should be able to delete it safely.
[UPDATE 5 March 2011: The latest updates to Firefox and Thunderbird have made their way into the Canonical repositories but not into Ubuntuzilla, as best I can tell. Message traffic on the Ubuntuzilla forum (from outsiders, not from project managers) suggests that Ubuntuzilla may have become inactive. So I've switched back to the "official" ports for Ubuntu. The process was easy -- just uninstall the old, install the new in Synaptic. I backed up my profile folders (~/.mozilla and ~/.thunderbird), but the backups were not needed in the end. Rumor has it Canonical has adopted a policy of sanctioning updates, at least for the Mozilla programs, more quickly. So thanks to the Ubuntuzilla folks for the service they provided while it was needed.]