User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:76.0) Gecko/20100101 Firefox/76.0

Steps to reproduce:

I installed 79.0b1 to see if it fixed a problem I'd been having

Actual results:

It offered to create a new profile instead of updating my existing one because it's not backward compatible. After 79.0b1 kept crashing and losing the message I was trying to send, I tried to revert and found that the stock thunderbird will no longer start because the original profile got upgraded anyhow.

Expected results:

I should have been able to revert to stock Thunderbird with no problem

That's expected.
You can go back, and it should not be a problem yet for 79, by specifying --allow-downgrade on the command line

Thanks, that did work