SSL / TLS has nothing to do with the thunderbird mail program …it has to do with your mail account settings
so if you have a mail account that is using SSL, like Gmail / Yahoo mail then you must use the correct setup if you want avast to scan the mail from those accounts
Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide communication security over the Internet.
Yes that is needed for avast to be able to scan the emails, but avast does handle the secure connection if it is required, so you aren’t losing out on that.