In the Options dialog, go to the Security panel. In the passwords section click the Exceptions button. You can then remove the site from the list that appears.

It does not show up in the exceptions box.

Make sure you are not in private browsing mode as that will disable saving of passwords.

You can try removing all site preferences. Select "Tools > Clear Recent History", make sure the only option selected is "Site Preferences", and for the time range select Everything. This will reset all of the exceptions.

For more details on that see the Delete browsing, search and download history on Firefox article.

The username and password not remembered article may also be of use.