If you do decide to go back, you need to start thundebird as follows: thunderbird.exe -p --allow-downgrade you can do that by clicking windows key and 'r' key together and then entering that text in the window that opens . be sure to back up the profile first. AND... it doesn't always work. There is some risk.

how to reverse version upgrade 91.12.0 back to former version 91.11.0

What are you trying to achieve in the first place?