--- a/toolkit/mozapps/extensions/LightweightThemeManager.jsm
+++ b/toolkit/mozapps/extensions/LightweightThemeManager.jsm
@@ -730,26 +730,26 @@ AddonWrapper.prototype = {
  *          The ID to be converted
  *
  * @return  the lightweight theme ID or null if the ID was not for a lightweight
  *          theme.
  */
 function _getInternalID(id) {
   if (!id)
     return null;
-  if (id == DEFAULT_THEME_ID)
+  if (LightweightThemeManager._builtInThemes.has(id))
     return id;
   let len = id.length - ID_SUFFIX.length;
   if (len > 0 && id.substring(len) == ID_SUFFIX)
     return id.substring(0, len);
   return null;
 }
 
 function _getExternalID(id) {
-  if (id == DEFAULT_THEME_ID)
+  if (LightweightThemeManager._builtInThemes.has(id))
     return id;
   return id + ID_SUFFIX;
 }
 
 function _setCurrentTheme(aData, aLocal) {
   aData = _sanitizeTheme(aData, null, aLocal);
 
   let cancel = Cc["@mozilla.org/supports-PRBool;1"].createInstance(Ci.nsISupportsPRBool);