Download perl script from http://batleth.sapienti-sat.org/projects/mb2md/#Download
Install perl-timedate (pkg name may vary by distro)
Make copy of .thunderbird MAIL
Run script on COPY of MAIL

$ mb2md-3.20.pl -s ~/mail-transition/Local\ Folders/ -d ~/mail-transition/maildir -R

Renamed generated folders to liking and then moved them to $MAIL dir