It's true some people are having issues with it but the major problems seem to be only on certain setups. There are some workarounds available for the Thunderbird issue, turning off email scanning for instance but you have a good point and sorry to see you go.
Technical Support are there to help you with this sort of thing and if the issue is escalated they will work through each issue with you to a happy conclusion, hopefully. They are free to contact by phone or online chat.
I have 12.1 installed on every OS that is currently supported and the only issue I've seen is a marginally slower startup and I know that they are aware of that. But then I don't use Thunderbird.
Thanks for posting and good luck ;-)