These are the normal steps:

Exit Thunderbird if running.
   click windows key and 'r' key
   Enter: thunderbird.exe -P --allow-downgrade
   click on 'OK' 

But that is no guarantee. Going backward in software releases is always a dance with the devil. As a volunteer with no skin in the game, I encourage you to stay the course. SuperNova is the way forward and it gets better with each new update.

Those are Windows-only instructions. What about on Mac?

The bugs in this release are unacceptable. It's going to have to make many "steps forward" from here just to get back to the working email client I had last week.