I can think of two possible causes. When you updated Thunderbird, the Firewall thinks it's a different program and has got itself confused. The Firewall might be the default one which came with OS or part of an Anti-Virus program. Exit Thunderbird Access Firewall Is Thunderbird set up as an allowed program ? If not set as an allowed program then you need to set up Thunderbird to be an alowed program. If it is already set up, then block/remove Thunderbird, ok it and then access Firewall again and reset to allow Thunderbird. Then Start Thunderbird. -------------------------------------------- The other possible reason could be xfinity blocking access. Xfinity introduced an additional level of security which means you had to logon to the webmail account and select the option to allow third party apps. Please check this. https://www.xfinity.com/support/articles/third-party-email-access