Tools > Options > Security > Saved Passwords. The list of usernames & passwords and the sites associated with them comes up. You should be able of editing them directly, but if not, just remove the entry and it'll ask if you want to save it again next time you use the site.
EDIT: Apparently you can't edit the passwords/usernames there... you'll need to delete the entry and then let FF save it again next time you visit the site.