update ui appropriately for helper apps (96939). r=hewitt sr=ben

xpfe/components/prefwindow/resources/content/pref-applications.js

@@ -107,6 +107,10 @@ function selectApplication()
     delete handlerOverride;
   } else {
     updateLockedButtonState(false)
+    gHandlerField.removeAttribute("value");
+    document.getElementById("contentTypeImage").removeAttribute("src");
+    gExtensionField.removeAttribute("value");
+    gMIMETypeField.removeAttribute("value");
   }
 }