said by Bobby_Peru:
Not sure from your description if Fx is auto-filling in your credentials, but I would try going to Options--> Security --> Saved Passwords
and looking for the site there.
If it is there, after making sure you have your current password saved somewhere else, highlight it, and click "Remove".
Then close all tabs related to this site, then clear all cookies related to this site, then clear your cache, then try going to the log-in page again.
Are you using any password extension, like Secure-Login? hth
How do I clear my cache?--
Go Mark Martin!