Yes you can install over the old version from http://archive.mozilla.org/pub/thunderbird/releases/78.14.0/mac/en-US/Thunderbird%2078.14.0.dmg

There is a missing update rule, which allowed updates on the unsupported OS.

Hi Wayne - thanks very much for your help. It took me a while to figure this out. The link you gave me is the application itself, without the installer. After some searching I learned from a couple of other sites (macupdate.com and techy.com) that I had to drag the icon from the .dmg folder into my Applications folder. As an 80-year-old technological dinosaur, I was terrified of losing my local folders which I use as an extensive filing system, and I poked around a good bit in Mozilla help to find out about Thunderbird profiles, how to find and save them, etc. In the end, when I finally got up enough courage to drag the new (old) Thunderbird icon into the Applications folder and told it to replace the existing (version 91) Thunderbird, it picked up my profile and version 78.14.0 now seems to be working fine for me, and version 91 is gone. I don't know if you are in touch with the people who write the updates and new versions, but I hope you will make sure that in future they include the "missing code" so that Thunderbird running on unsupported systems will not be updated. At least now I know not to update Thunderbird any more, and what to do in case of a disaster. For the time being I am not updating Firefox either. But for a while there I was wishing I knew how to put a witch's curse on somebody, so I could put it on whoever left out that piece of code.

Thank you again, - Miriam