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diff --git a/xpcom/threads/nsISerialEventTarget.idl b/xpcom/threads/nsISerialEventTarget.idl
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+++ b/xpcom/threads/nsISerialEventTarget.idl
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "nsIEventTarget.idl"
+
+/**
+ * A serial event target is an event dispatching interface like
+ * nsIEventTarget. Runnables dispatched to an nsISerialEventTarget are required
+ * to execute serially. That is, two different runnables dispatched to the
+ * target should never be allowed to execute simultaneously. One exception to
+ * this rule is nested event loops. If a runnable spins a nested event loop,
+ * causing another runnable dispatched to the target to run, the target may
+ * still be considered "serial".
+ *
+ * Examples:
+ * - nsIThread is a serial event target.
+ * - Thread pools are not serial event targets.
+ * - However, one can "convert" a thread pool into an nsISerialEventTarget
+ *   by putting a TaskQueue in front of it.
+ */
+[builtinclass, scriptable, uuid(9f982380-24b4-49f3-88f6-45e2952036c7)]
+interface nsISerialEventTarget : nsIEventTarget
+{
+};