Thunderbird 78 has just hit Amber today. The Enigmail extension has been deprecated as the Thunderbird team (and Enigmail dev) have now built PGP encryption directly into Thunderbird itself.
One thing folks may run into is if you have moved your encryption keys off your computer and onto your Librem Key (or any other smartcard, hard token, etc), those will not work out of the box with Thunderbird 78. See here for more info: https://wiki.mozilla.org/Thunderbird:OpenPGP:Smartcards
Also this article (https://lwn.net/Articles/832183/) was posted internally by a team member and may be of help to some.
If folks have questions or good tips/tricks to help with this transition, please go to Enigmail slated for removal? and discuss it there.
Edit: PureOS Wiki Entry: https://tracker.pureos.net/w/troubleshooting/tb78gpgkey/