I suggest you update Thunderbird to the latest version. In the mean time disable email scanning in your anti virus product of choice. Some anti virus products do not do POP email very well at all really.

did not solve the problem. Only one of 5 accounts affected by this problem

I have seen hundred of folk saying the same group of emails, or the same email over and over on a pop mail account. It has always been a malformed email (generally in a spam folder on the mail server) that causes the antivirus to just choke and die. The result is the mail already downloaded in the session is not recorded as such and every future attempts has exactly the same result. The mail in the queue before the malformed one download (even if it is one mail) and then the process dies a horrible death.

So empty the spam folder on comcasts web site as turning off your anti virus mail scanner did not apparently do that.