Today, I deleted my Mailfence account from TB, restarted TB, and then tried to add Mailfence. TB responds with "Unable to login at server."

I went to Thunderbird.net and downloaded the latest version, and I got 139.0.2. I just got an update that made my TB 128.11.1. Launching the new version did not change anything. TB still will not connect to Mailfence.

Do you have an Entry or higher subscription? Only those types allow IMAP, POP and SMTP access.

Yes, I do. I can get in to my Mailfence account just fine with Firefox, and the Mailfence app on my phone.

What is the antivirus? Using a VPN? Try running Windows in safe mode (which bypasses those apps) to test for AV or VPN interference.

Norton 360. Checked settings, doesn't know TB exists. TB does connect to my one Outlook account and two Gmail accounts just fine. I uninstalled TB, restarted computer, installed 139.0.2, restarted computer, launched TB, still the same.

Norton is a common source of blockages, which is why it's suggested to run Windows in safe mode, or remove Norton (disabling it is not sufficient).

https://support.mozilla.org/en-US/questions/1485137

I looked at the question the link took me to. The user said their problem affected their Gmail accounts and posted a screenshot of a dialog box about certificates when sending encrypted emails. I get no dialog boxes, notices, alerts, etc. Just what's in the lower left corner of the pane. Every ten minutes when TB checks my accounts for new messages, the status messages are displayed in the lower left of the TB pane. For Mailfence only, it will say "connected to imap.mailfence.com..." and it stays there for about five seconds and then disappears. No error messages , boxes, etc.

I looked at the question the link took me to. The user said their problem affected their Gmail accounts and posted a screenshot of a dialog box about certificates when sending encrypted emails. I get no dialog boxes, notices, alerts, etc.

Just get rid of Norton like you were asked to, instead of delaying the inevitable repair or diagnostic result removing it will create. Then you can contact Norton for setting for their virus like product that actually work for you. If you think I have a strong personal distaste for Norton, you would be correct. My last paid subscription was some 20 years ago and the longer I hang around support forums the more sure I am that dumping them was a good decision.

I "got rid of Norton", like you said. After restarting Windows, with NO Norton, I launched TB and IT STILL STOPPED at "connected to imap.mailfence.com". Does not login to Mailfence. Thank you for your "expert" advice. I have uninstalled Thunderbird and cancelled my recurring donation.

Quite perplexing. And I wasn't aware of Larry's history but when I searched for mailfence problems https://support.mozilla.org/en-US/questions/1450842 is among the first to come up. And in that issue, it looks like for unknown reasons it solved itself?

In that instance, TB broke after an update. After many attempts to repair, I removed TB and went back to an older version. After numerous restarts for updates it got back to the latest version and worked. In this instance, Outlook and Gmail accounts connect normally. Only the connection to Mailfence stalls. It will not login. Connection History at Mailfence shows a valid connection, but doesn't show from what. I know it's TB , because one connection is DAV, the address book. After reverting to a previous version of TB, uninstalling a Windows update, that happened right before this started, and checking the configuration for Mailfence at least 15 times, I am checking with Mailfence to see if it's on their end. I knew Norton was NEVER the problem, but I humored the arrogant Moderator, and now it's cost Thunderbird a longtime user/donor. Sorry.