See if this helps: https://support.mozilla.org/en-US/questions/1466258

The replies attempting to address my problem do not tell me exactly what I need to do in order to access the Comcast email system using Thunderbird. The answer my be somewhere in the technical terms and jargon, but I am 80 years old and one of the Multitude of the Unwashed, so I am not privileged to be able to comprehend the information . Thunderbird worked for many years on my old computer. I cannot load or use it on my new (to me) computer. So far, Comcast seems to be blaming Thunderbird for the problem and Thunderbird is blaming Comcast. Someone out there must have the answer. Please let me know. Thanks, John

Please post screenshots of your incoming server settings and your SMTP server settings. Thank you.