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; },