Yes. That's my post on forums.xfinity.com. My Thunderbird 140.2.0esr was updated 8/19/2025 at 8:35:37 PM (local?) per the Troubleshooting info Update History. This matches the time (within 2 secs) on the various files in the C:\Program Files\Mozilla Thunderbird folder. Last used (before today) 8/26/25 at 5:55pm EDT with no problems, per the file time on the cache files in C:\Users\peter\AppData\Local\Thunderbird\Profiles\8bsv5n66.default-release\cache2\entries for the 26th. I shutdown the app by the end of the day if not sooner.

Anyone have a way to trace the comcast POP3 login process used by Thunderbird to see where the failure occurs? Note, have since seen another error once or twice when starting up Thunderbird, something about connection being reset.

Comcast has appeared to have resolved the problem.

As of today, the Comcast problem is still occurring for me. I cannot receive email and am given an error message to retry or reenter my password. Upon entering the correct password, it simply gives me the same error.

Thunderbird was trying to pop open an Xfinity authentication window, then failing with errors like "https://localhost/?code=0.ac.w2.7viWYX" without ever displaying the login window.

What solved the problem for me was deleting cookies, clicking Display the Thunderbird Menu (the hamburger icon of 3 horizontal lines at the top right), Privacy & Security, Web Content section, click Show Cookies, Search:, start typing "xfinity.com" to find all the oauth.xfinity.com, login.xfinity.com, and xfinity.com cookies (click the top one, scroll down to the bottom, shift-click the last one to select all of them), then Remove Selected. I closed and relaunched Thunderbird, and immediately the login window was able to open, and I could enter the Comcast.net email address, then password, then 2-factor authentication code.