diff -r 000000000000 -r 6474c204b198 xpcom/base/nsIMutable.idl
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/base/nsIMutable.idl	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,22 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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/. */
+
+#include "nsISupports.idl"
+
+/**
+ * nsIMutable defines an interface to be implemented by objects which
+ * can be made immutable.
+ */
+[scriptable, uuid(321578d0-03c1-4d95-8821-021ac612d18d)]
+interface nsIMutable : nsISupports
+{
+    /**
+     * Control whether or not this object can be modified.  If the flag is
+     * false, no modification is allowed.  Once the flag has been set to false,
+     * it cannot be reset back to true -- attempts to do so throw
+     * NS_ERROR_INVALID_ARG.
+     */
+    attribute boolean mutable;
+};