i no longer use Thunderbird, but as far as i remember on the left you see your email accounts. If you right click on your email account in the left pane and click properties it should take you there.
As its a POP3 account also turn IMAP off if you have it seleted. Although i wasn't aware you could even select both. My guess is that you either have the wrong port numbers, or Avast has started to block them. This only happened since last update, yes?