Serving more of a bookmark, these are the best set of extensions for Thunderbird I am currently using:

Mail Redirect (for the “bounce” effect a la mutt):
https://addons.mozilla.org/en-US/thunderbird/addon/550

Quote Collaspe (to reduce the annoying top posters):
https://addons.mozilla.org/en-US/thunderbird/addon/347

Enigmail (for GPG/PGP signing):
https://addons.mozilla.org/en-US/thunderbird/addon/71

And providing the same GPG/PGP goodness for Gmail through FIrefox:
http://firegpg.tuxfamily.org/