I think you'll be well protected if you just make a backup copy of the subdirectory ~/.thunderbird (at least on my distro). I know the message databases are in daughter subdirectories of ~/.thunderbird, and I believe the accounts settings and other preferences are stashed down there, as well.
Tbird may be organized differently on your distro, but it shouldn't be dramatically different.
I've upgraded and downgraded Tbird several times and have never lost anything in the process, but I always make a message base backup first, just in case.
Last edited by dhave; 12-15-2004 at 05:16 PM.