Can't Login to comcast.net account with Tthunderbird

          Login to server imap.comcast.net with username dick-lincoln@comcast.net failed.

I can't Login to comcast.net account with Thunderbird. I keep getting the following error message: Login to server imap.comcast.net with username dick-lincoln@comcast.net failed. I CAN login to that account through Firefox using the same credentials as I input to Thunderbird. I have been using TB on that account for several weeks now successfully, but now it fails even after rebooting my computer. During that time nothing has changed in my comcast.net credentials. The server address is correct for receiving emails.