Enigmail

Enigmail is a security extension to Mozilla Thunderbird and Seamonkey. It enables you to write and receive email messages signed and/or encrypted with the OpenPGP standard.
Sending and receiving encrypted and digitally signed email is simple using Enigmail.
When starting it for the first time, you are guided through the basic setup. We also prepared a new users' guide that explains how to use OpenPGP.

http://enigmail.mozdev.org