Link: https://enigmail.mozdev.org/home/index.php

It's old news but everyone should be using it.
So here goes another reminder.

Enigmail is a security extension to Mozilla Thunderbird and Seamonkey. It integrates the renowned OpenPGP standard provided by GnuPG.

Sending and receiving encrypted and digitally signed email gets just simple using Enigmail.

Secure Email-Clients with PGP/MIME
http://www.bretschneidernet.de/tips/secmua.html