Are there any (possibly empty) names and passwords for that site listed in the Password Manager?

• Firefox > Preferences > Security: Passwords: "Saved Passwords" > "Show Passwords"

Did you check the password exceptions?

Make sure that you do not run Firefox in permanent Private Browsing mode.

• https://support.mozilla.com/kb/Private+Browsing

To see all History and Cookie settings, choose:

• Tools > Options > Privacy, choose the setting Firefox will: Use custom settings for history
• Deselect: [ ] "Always use private browsing mode"

Been there, checked that. It's a bank site, and they have separate logins for regular banking and credit cards. I disabled the regular banking (the one I use the most and want to restore), but not the credit card one (which still works). But no blank fields for that site, and it's not listed in the "exceptions."

The website may be using autocomplete=off to prevent Firefox from saving the name and password.

You can remove autocomplete=off with a bookmarklet to make Firefox save the name and password.

• http://kb.mozillazine.org/User_name_and_password_not_remembered

• Saved Password Editor: https://addons.mozilla.org/firefox/addon/saved-password-editor/