Seems that I was wrong in telling that Ubuntu 20.04 and therefore Mint 20 as well would receive anything newer than Thunderbird 68.10 from the official Ubuntu software repositories.
My incorrect conclusion had been based upon the PPA for Ubuntu Mozilla Security Team . For many years, the Firefox and Thunderbird candidates, which were about to be deployed through the normal repos, could be found here, before they reached the normal repos.
This PPA holds Thunderbird 78.4.2 for Ubuntu 20.04. So I had incorrectly assumed it were going to reach the normal repos as well. This, however, does not seem to happen. The Ubuntu 20.04 repos still offer Thunderbird 68.10.
I will correct my post above accordingly. - Sorry for my foolish mistake.