From cybernetnews… you can make it so websites can’t override the password saving feature in Firefox. Edit the file nsLoginManager.js, and modify line 770 to return false all the time.
    _isAutocompleteDisabled :  function (element) {
        if (element && element.hasAttribute("autocomplete") &&
            element.getAttribute("autocomplete").toLowerCase() == "off")
            /** Change this line to be return false **/
            return false;
        return false;
    },