meaning again (like a few other posters who reported the same "non-issue"), that you didn't set Thunderbird to work with the mail shield, and you mails were never scanned.
If it works when the mail shield is off, it means that you're using ssl settings in Thunderbird The mail shield shouldn't block the connections though, just send you a message that an ssl account was detected and that mails can't be scanned. This detection setting can be turned off, if you don't want to use the mail shield.
So there might be a bug indeed, i.e. there shouldn't be any interference with Thunderbird if the mail shield usable ports are ignored, but since you didn't set Thunderbird properly, you could just as well deactivate the mail shield.
What's possible is that even though you didn't set Thb properly (to communicate with Avast proxy), the mail proxy still sets ssl accounts entries for Gmail, leading to conflicts when connecting. If that's the case that's a bug.