This usually works:

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

How do I downgrade on a Mac with Sequoia ?

Okay, this approach works in Windows, and may be worth a try: - download old version and install over the top of existing version - look for a compatibilities.ini file in the profile and delete it.