FYI: Since the update 68 → 78 removes Enigmail (PGP is now included in Thunderbird itself), Thunderbird provides a migration wizard under “Tools → Migrate Enigmail settings” that will recover all things Enigmail and convert them to the new system, so you don’t loose your keys/settings.
You’ll need to update Enigmail to 2.2 after updating to TB78 (installable via Addons) for that. See also here.
Double-check that you have the Enigmail v2.2 plugin installed? The way I understood it all migration functionality is contained in that. TB should have updated it automatically for you, but maybe that didn’t happen.
Maybe file a bug for that? TB 78 is now in all repos except F31 (where it is in Testing), so thunderbird-enigmail-2.1.whatever will no longer do anything & everybody who uses that package will run into your problem.
Sorry to ask (dfferent topic), do we already see a meaningful migration description for enigmail users to tb 78? I have/had a working tb68 + enigmail config. And when upgrading to 78, I found no meaningful way to import my keyrings. public keyrings did not accepted for import, private keyring import did not let me select specific keys but seems to require I have on me the information to all the private keys that I ever have used.
I can confirm this worked.
Note: You must remove thunderbird-enigmail if it had been installed from Fedora repo (which for me was the case). (then update to tb78, then install enigmail from the addons store, …)