Font size and multiple other layout parts. I am using Thunderbird on 3 operating systems and adding new account is done in different places in all three up-to-date Thunderbirds. For several months now, Thunderbird tends to ignore manually inputted IMAP host address and tries to use x part of y@x e-mail address format to connect to, even if there's no A/AAAA records for it (MX exists). e-mail-related SRV records seem to be ignored.

Regarding font size, it can be changed with mouse scroll even in windows like account configuration.