You can delete a saved username/password pair, but if the problem occurs repeatedly, I wonder whether the site is designed to frustrate automatic logins?

To clear the existing saved login:

orange Firefox button or classic Tools menu > Options > Security > "Saved Passwords" button

You can check if there are entries in the password manager with an empty user name.
Make sure that you haven't saved multiple (uppercase, lowercase) versions of that name and password.

• Tools > Options > Security: Passwords: "Saved Passwords" > "Show Passwords"

You can remove all stored data from a specific domain via "Forget About This Site" in the right-click context menu of an history entry (Show All History or History sidebar) or via the about:permissions page.

Using "Forget About This Site" will remove everything like bookmarks, cookies, passwords, cache, history, and exceptions from that domain, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks.
You can't recover from that "forget" unless you have a backup of the affected files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

Problem solved.

"You can remove all stored data from a specific domain via "Forget About This Site" in the right-click context menu of an history entry (Show All History or History sidebar) or via the about:permissions page."

Many thanks, that worked, where as just deleting the site from the password manager didn't. Must have been due to using a saved profile when I reinstalled Firefox. The profile must have been somehow corrupt.

I wonder why the same profile worked OK in Linux?

Such issues can also be caused by cookies that aren't working properly or only may work on a specific platform if you did restore those as well.

• https://support.mozilla.org/kb/fix-login-issues-on-websites-require-passwords