GNU Privacy Guard (GnuPG) is "a complete and free implementation of the OpenPGP standard as defined by RFC4880."

OpenPGP is "the most widely used email encryption standard. It is defined by the OpenPGP Working Group of the Internet Engineering Task Force (IETF) as a Proposed Standard in RFC 4880."

OpenPGP in Thunderbird: HOWTO and FAQ

This article provides detailed information for users of Thunderbird who want to send and receive encrypted and digitally signed email messages using the OpenPGP standard. This feature is commonly known as end-to-end encryption (e2ee), and makes communications safer against being spied on by third parties. Thunderbird 78 has built-in support for two encryption standards, OpenPGP and S/MIME.

My public key is available for download. The fingerprint is: 1D2EED2F9D906875C4F4DAD2068C9178672BA80E