Enigmail is an appealing and fairly simple to use piece of software designed to function as a Mozilla Thunderbird and Seamonkey extension which provides you with OpenPGP encryption and authentication capabilities, so you can protect the contents of the messages you send and receive.
Donationware
Whats new:>> As of Thunderbird version 78, OpenPGP is directly integrated in the mail client. Enigmail is not required anymore.
This version of Enigmail serves solely for migrating your keys and settings from Enigmail/GnuPG to Thunderbird. If you did not use Enigmail in Thunderbird 68 or earlier, then this version of Enigmail will not do anything.
https://www.enigmail.net