FYI, solved it. I use HTML files as signatures. Apparently the HTML of the standard email account was not liked by the new Thunderbird. Setting up a new HTML signature did the trick.

what was the actual issue with the signature file?

Not sure, but there was some pretty wild CSS styling. I think I used a letterhead in Word as basis and then converted to HTML. This did not keep it simple and elegant at all.

That sort of explains it. Word XML and binary objects in HTML can actually get your mail treated as SPAM and send anti virus programs off in a loop.

The steps here are very clear and simple. I have not idea how to simplify them https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-and-import#w_viewing-and-deleting-passwords

To remove the username and password for a website, select the site's entry from the list and click Remove.

I have no idea how things are set up with your provider, but it might be that you simply can not do what you want if the mail and the site are opening on the same domain name. you will just have to remove the site you think it is and see if you are asked for a password. This is not technical stuff. IT is the same as sorting through a bundle of paper notes and trying to work out which one has the right phone number on it based on an address.