Disabling AV and/or safe mode does not help

What is your anti-virus software?

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

The issue is dependant on incoming email. I need to wait until I have email incoming to test this. I will see if I can get a screen capture of the problem in progress. It may yield more information than my description.

Win10 safe mode is extremely annoying. I almost hosed my computer using safe mode because it would not accept ANY passwords for MS account nor admin account. Luckily I found out how to undo the setting. The password(s) I entered were right....dumb windows.

I will see if I can get a screen capture of the problem in progress.

I somehow doubt that a screenshot will get us any further.

I almost hosed my computer using safe mode because it would not accept ANY passwords for MS account

Windows safe mode does not change the way how Windows login works. So I'm not sure what you did, and why that was a problem. You may want to ask in a Windows forum to sort this out first.

Windows safe mode prevents certain software such as anti-virus from being started. Therefore Windows safe mode is not a cure, but a test which will help to determine whether anti-virus software is the culprit. Please report back your results.

Windows safe mode disabled using PIN log in and instead asked for a password. But I used both my admin password and the microsoft account password. Neither worked. I was lucky to have internet on my phone to find the way to undo the safe mode option set in msconfig. I had to use bcdedit in the command prompt from the recovery console.

Yes. I'm aware of what safe mode does. I've been working with windows computers since Windows 3.11 for workgroups in 1994. My first computer had Windows 95 OSR2.

Windows 10 safe mode is a pill. Wish they would go back to the press F8 for options and allow a way to return to normal mode without doing what I had to do. If I could not have used bcdedit. I would not have been able to access my computer. What is odd is that the password I was using WAS the correct log in for my MS account which is what it was prompting for.

The problem was that windows doesn't say which password is wants. There are two an admin password, a user account password, a four digit pin and the MS account password.

No matter. For some reason I have not experienced the issue today with thunderbird gathering email. The only thing I have done was to reset my tool bar in the menu that allows thunderbird safe mode.