See https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-tb

Thanks, I got almost there, but I need a little more help. I got to the list of accounts and clicked on "show passwords". It asked for a PIN. I don't know the PIN. It said that I could reset the PIN under Settings/accounts. I didn't see that exact thing, but under Settings I saw Account Settings, but I didn't see how to set the PIN there.

How can I set the PIN?

That's strange. Mine asks me for the password (a mixture of characters), not a PIN (numbers only). But you can set up a PIN for access into the Windows OS which, I guess, is what you must have done and it's this that you would use to log in at the lock screen. This is what you would enter in the TB field asking for your PIN.

See https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-primary-password

Although the view password button pops out to the operating system for your account password/Pin. As do not have any password on my user account, and do not mess with the Microsoft online mess that is what windows login has become, I do not get asked for anything. On my phone I am asked for a thumbprint as that is my unlock bio-metric on my phone.

I might guess you are getting a pin from Microsoft on your phone that you need to use. But without extensive knowledge of what security you have allowed and what privacy invasions you have permitted encouraged by Microsoft and their deceptive login dialogs there is no real way to guess what your operating system password/PIN is. Needless to say you must be using it in other places like office365 so you should know what the procedure is.