Bugzilla

I just had the same issue with an ancient IMAPS server (courier imap), and also found this bug 1669676 closed as invalid.   (and solved it by setting security.tls.version.min=1)

I think the main issue is the lack of a specific error message. 
No problem with requiring by default TLS1.2, but there should be an error message stating the problem when connecting to an older server.

With old accounts there is no indication that something is wrong, it even shows "connected to ..." message in the lower bar, even if nothing is happening. But I just had messages till  the last upgrade of TB. 
I also tried enabling debug logs of imap but just got a generic "URL failed with code 0x805a2ff7" 

I also tried deleting and recreating the account,  which failed with the message "unable to log in at the server. Probably wrong configuration, username or password" which is not very helpful.