You don't think you can just use a secure connection unless your ISP supports it. Some ISPs insist on it and then you are forced to use it.
If you then changed your settings in tbird to TLS or SSL then avast 'can't' possibly have worked as it doesn't (can't) scan secure connections, that is the whole point of a secure connection to keep prying eyes out.
These are commonly the secure ports - SSL/TLS (Secure) Ports POP3 995; SMTP Ports, Thunderbird-587, If using these ports avast doesn't monitor them. If you try to use the standard ports 110, 25, etc. and use secure communication with them then avast intercepts those ports but can't scan the encrypted content and it usually fails to upload as it doesn't know how to send to a secure connection.
So I'm even more confused, but that happens easily