In FreeBSD, the -rpath for the linker does not include the area where thunderbird's libs are located. Thunderbird works, but not some of the pieces. This is picked up in revdep-rebuild, but it will just keep building it over and over each time revdep-rebuild is run. This also causes enigmail to be rebuilt over and over via revdep-rebuild.
Created attachment 135986 [details, diff] files/mozilla-thunderbird-2.0.0.6-freebsd.patch This patch fixes it (and also fixes enigmail). Let me know if OK to check in.
P.S. This patch was based on a similar one for firefox, found here: http://sources.gentoo.org/viewcvs.py/gentoo/src/patchsets/mozilla-firefox/2.0.0.8/803_fbsd-3.patch?view=markup
Patch added to thunderbird-2.0.0.9, patchset 0.1