I think it would be helpful if it was possible to add custom fields (or a textarea for notes) to stored passwords to securely store additional information (like security questions, customer support pins, etc.)
Additionally it would be nice to be able to store and sync other credentials and information in Firefox, that don't link to websites, like (recovery-)keys for BitLocker/TrueCrypt/LUKS-LVM partitions, WiFi Passwords and Credit Cards. Of cause you won't have auto-fill, etc., but it wouldn't cause any harm to have more like a "Firefox secure wallet"...