regarding last version of Thunderbird when I want to add email account , now password authentication method - Oauth2 - is not proposed , then no possibility of connection to yahoo server and then impossible to add my account !
Anyway on other Yahoo existing accounts , Oauth2 option is well configured ! also I remove Thunderbird and reinstall but nothing more .
New accounts that you create with a mail service?
Because I just add “anew” (from scratch on a newly installed Thunderbird) a couple of I have had with some mail service and it worked.