diff -r 000000000000 -r 6474c204b198 content/base/src/nsStubMutationObserver.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/base/src/nsStubMutationObserver.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,34 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * nsStubMutationObserver is an implementation of the nsIMutationObserver
+ * interface (except for the methods on nsISupports) that is intended to be
+ * used as a base class within the content/layout library.  All methods do
+ * nothing.
+ */
+
+#ifndef nsStubMutationObserver_h_
+#define nsStubMutationObserver_h_
+
+#include "nsIMutationObserver.h"
+
+/**
+ * There are two advantages to inheriting from nsStubMutationObserver
+ * rather than directly from nsIMutationObserver:
+ *  1. smaller compiled code size (since there's no need for the code
+ *     for the empty virtual function implementations for every
+ *     nsIMutationObserver implementation)
+ *  2. the performance of document's loop over observers benefits from
+ *     the fact that more of the functions called are the same (which
+ *     can reduce instruction cache misses and perhaps improve branch
+ *     prediction)
+ */
+class nsStubMutationObserver : public nsIMutationObserver {
+public:
+  NS_DECL_NSIMUTATIONOBSERVER
+};
+
+#endif /* !defined(nsStubMutationObserver_h_) */