T'bird can be a little confusing to make changes to server settings...
Here's the page from Mozilla explaining how to do it:
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support.mozillamessaging ··· gurationIf you have an A/V program (e.g. Avast) that does SSL, then that's where you would set the protocols for the servers - letting it handle the SSL work rather than doing it in the email client.
I'm kinda old school and have been using Eudora forever. I find it less complicated than T'bird...
Speaking of which, has anyone noticed that V's email servers have been a little wonky the past couple of days? I keep getting disconnects and timeouts on the new ones.