I had no problems transitioning from Enigmail to Thunderbird 78.3.1,
which has removed Enigmail. With an existing GPG installation, it
was necessary to run the command "gpg --export-secret-keys --armor >
private_key.asc" for importation into Thunderbird. Then in Thunderbird,
clicking the main account header in the side panel brings up a link
"End-to-end encryption." This brings up "Account Settings," toward the
bottom of which is an "End-To-End Encryption" header. From here one can
enable OpenPGP by importing the "private_key.asc" file created earlier.