Windows password has nothing to do with the site login. Does your Windows Password have a Hotmail or Outlook login? If so I always advise to not use your Email account as your login for Windows 10 it is a pain in the a55 and not secure as if you forget or lock yourself out your locked out of Windows 10. I don't setup my Windows 10 login to any email accounts for that reason. Also posting in english would help to get faster help as well since this is english forum.

Hi WestEnd, thanks for your answer. I do not not use an email-account or MS Live Essentials for the login. But I found entries in “Control Panel\User Accounts\Credential Manager” and deleted them, since this date no problems with Firefox anymore.