Synching.

I was wrong, it is possible.
Instructions are located at http://maienm.heliohost.org/tb/tbpsync.html
USE AT OWN RISK!!!!
EXIT ALL INSTANCES OF THUNDERBIRD BEFORE USING THIS METHOD
Not thoroughly tested, but it seems to work fine from the tests I ran.
Oh, and this won't sync your extentions, just your mail (and settings?).

Please give feedback.