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diff --git a/js/xpconnect/idl/nsIAddonInterposition.idl b/js/xpconnect/idl/nsIAddonInterposition.idl
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+++ b/js/xpconnect/idl/nsIAddonInterposition.idl
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+/* -*- Mode: C; tab-width: 8; 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"
+
+/**
+ * This interface allows Firefox to expose different implementations of its own
+ * classes to add-ons. Once an interposition is created, it must be assigned to
+ * an add-on using Cu.setAddonInterposition (JS) or xpc::SetAddonInterposition
+ * (C++). In both cases, the arguments should be the add-on ID and the
+ * interposition object (which must be an nsIAddonInterposition). This must
+ * happen before any compartments are created for the given add-on.
+ *
+ * Every time the add-on accesses a property on any object outside its own set
+ * of compartments, XPConnect will call the interposition's
+ * interpose method. If the interposition wants to replace the given
+ * property, it should return a replacement property descriptor for it. If not,
+ * it should return null.
+ */
+[scriptable,uuid(d05cc5fd-ad88-41a6-854c-36fd94d69ddb)]
+interface nsIAddonInterposition : nsISupports
+{
+    /**
+     * Returns a replacement property descriptor for a browser object.
+     *
+     * @param addonId The ID of the add-on accessing the property.
+     * @param target The browser object being accessed.
+     * @param iface The IID of the interface the property is associated with. This
+     *              parameter is only available for XPCWrappedNative targets. As
+     *              such, it's only useful as an optimization to avoid
+     *              instanceof checks on the target.
+     * @param prop The name of the property being accessed.
+     * @return A property descriptor or null.
+     */
+    jsval interposeProperty(in jsval addonId,
+                            in jsval target,
+                            in nsIIDPtr iface,
+                            in jsval prop);
+    /**
+     * We're intercepting calls from add-ons scopes into non-addon scopes.
+     *
+     * @param addonId The ID of the add-on accessing the property.
+     * @param originalFunc The function object being intercepted.
+     * @param originalThis The |this| value for the intercepted call.
+     * @param args The arguments of the original call in an array.
+     * @return The result of the call. NOTE: after the call interception,
+     * the original function will not be called automatically, so the
+     * implementer has to do that.
+     */
+    jsval interposeCall(in jsval addonId,
+                        in jsval originalFunc,
+                        in jsval originalThis,
+                        in jsval args);
+
+    /**
+     * For the first time when the interposition is registered the engine
+     * calls getWhitelist and expects an array of strings. The strings are
+     * the name of properties the interposition wants interposeProperty
+     * to be called. It can be an empty array.
+     * Note: for CPOWs interposeProperty is always called regardless if
+     * the name of the property is on the whitelist or not.
+     */
+    jsval getWhitelist();
+};