I recently decided to use Thunderbird, on multiple PC’s, in IMAP mode (versus POP). To facilitate the ease of duplicating my Thunderbird and Firefox set up’s, on multiple PC’s, I backed up the settings, using MozBackup (http://mozbackup.jasnapaka.com/), and then restored the settings to the other PC’s.
Note you can simply use this tool to back up your settings, in case of an emergency. I back mine up to an external drive, for the day when (not “if”) my hard drive crashes.
Make sure you are running the same versions of Thunderbird and Firefox on each PC to ensure compatibility among your instances.
UPDATE (2011/08/05): I have successfully used MozBackup across Thunderbird and Firefox versions, and it works great. At least as of Thunderbird 5 and Firefox 5, MozBackup works fine going back to at least version 3 of each of the two applications.