I have been using Thunderbird for a very long time on both Gmail and AT&T (but not on Xfinity) and have never had to change any setting since my initial setup.
Big tip ! Create a folder under Local Folders called Drafts. Then right click on you account name in the left hand column, click Settings and click Copies and Folders. Change Keep Draft Messages in : Drafts on Local Folders.
If you have no Internet connection or Xfinity is down, you can still create (but not send) messages. Just remember to send them when you have a connection to Xfinity.