Did you enable 3rd-party access to the account?

https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email

Although I can't find the "3rd-party access" setting at this time, I'm sure I enabled it before because everything was working fine for a long time. However, sometimes Gremlins change settings in the dark of night, so I'd be happy to check on it and make sure it is enabled if I could find it. Thanks.

There's a link on the page cited above:

https://www.xfinity.com/support/articles/third-party-email-access

If it's already enabled, post any error message you see.

I found the setting (thanks for the instructions). 3rd party access was enabled. So I unchecked the box and then checked it again to force enable. Then I shut down Xfinity and started it up again to make sure the setting "took". 3rd party access was enabled. There aren't any error messages, but what Thunderbird shows is that it is trying to connect to the IMAP server...... and it keeps trying and trying.

If it was working until a TB update, it could be caused by an incompatibility with an antivirus app. Or, possibly downloads are being blocked by a corrupted message:

https://support.mozilla.org/en-US/questions/1158670

I had the same problem. After changing password for xfinity, I could not get any email updates, even after changing it in TB. my solution was to delete the account and reinstall it using the autosetup. After typing in the new password and testing, the connection tested good and all my emails returned. I just hope I don't have to do this everytime I change my password.