That sounds good, EXCEPT, how to I tell Avast 5 to stop giving me these:
. . . ?
sounds good? that just means that you can keep sending emails but avast won't scan them unless you change your settings in Thunderbird. Just set your port to either 25 or 587, no ssl and you're done. As to your authentication issue, it just means that your mail server requires authentication, but not necessarily a secure authentication, nothing to do with the ssl connection required to send and receive
So, in Thunderbird, authentication just means that you've filled up the username box, don't add @xxx.com, it's not needed, but put everything else exactly how it appears in your email address, again, without the @ and the domain. If your server required a secure authentication, there's a check box for this below the username that you entered, but I doubt you need it.
ps: otherwise, if you don't mind avast not scanning, you can deactivate the warning about ssl somewhere in the email scanner settings.