I was just giving thunderbird 81 beta a try to see how the built-in PGP support works, and have some (hopefully useful) feedback.
First, it would be nice if the “Import public keys…” file requester would be a multi-file selector instead of single file. I have a whole bunch of .asc files with keys and it is a laborious process to import them individually. I did try concatenating them all into one big .asc file, but the key manager just imported the first one and stopped.
Also, I don’t know if I haven’t looked hard enough, but I can’t seem to find any options to add local key servers to the key manager.
Finally, GMail corporate users can enable S/MIME encryption for internal mail, but if someone is using Enigmail to encrypt the message before sending through GMail, GMail’s additional processing when S/MIME is enabled changes the message in such a way that it causes Thunderbird 81 to crash and bomb out. Note that Enigmail also couldn’t decrypt mails that have been through the Enigmail -> GMail S/MIME scheme, but Outlook with Symantec PGP Desktop can read them fine.
But otherwise, having built in PGP is really nice and looks good so far.