The ability to upgrade from Thunderbird 68 to the latest, version 78, was initially held back for a long time due to issues with Enigmail going away and being replaced with OpenPGP, and OpenPGP wasn’t quite ready.
I believe as of late September, it’s “ready” and you can upgrade. Windows users have the option to upgrade within the app (Help | About), which suggests that it’s ready for prime time.
I’ve not seen 78 in the official repository yet. Anyone know what’s the hold up?