It is a pretty rare event to see a nearly 21-year-old bug be addressed—many
projects are nowhere near that old for one thing—but that is just what has
occurred for the Mozilla Thunderbird email
application. An enhancement
request filed at the end of 1999 asked for a plugin to support email encryption, but it has mostly
languished since. The Enigmail plugin did come
along to fill the gap by providing OpenPGP support using GNU Privacy Guard (GnuPG or GPG), but was
never part of Thunderbird.
As part of Thunderbird 78,
though, OpenPGP is now fully supported within the mail user agent
(MUA).

Source: LWN.net – [$] OpenPGP in Thunderbird