--- a/layout/style/StyleSheet.h
+++ b/layout/style/StyleSheet.h
@@ -139,15 +139,15 @@ protected:
   nsIDocument*          mDocument; // weak ref; parents maintain this for their children
   nsINode*              mOwningNode; // weak ref
 
   // mParsingMode controls access to nonstandard style constructs that
   // are not safe for use on the public Web but necessary in UA sheets
   // and/or useful in user sheets.
   css::SheetParsingMode mParsingMode;
 
-  StyleBackendType      mType;
+  const StyleBackendType mType;
   bool                  mDisabled;
 };
 
 } // namespace mozilla
 
 #endif // mozilla_StyleSheet_h