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diff --git a/xpcom/threads/nsMemoryPressure.h b/xpcom/threads/nsMemoryPressure.h
new file mode 100644
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+++ b/xpcom/threads/nsMemoryPressure.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef nsMemoryPressure_h__
+#define nsMemoryPressure_h__
+
+#include "nscore.h"
+
+enum MemoryPressureState
+{
+  /*
+   * No memory pressure.
+   */
+  MemPressure_None = 0,
+
+  /*
+   * New memory pressure deteced.
+   *
+   * On a new memory pressure, we stop everything to start cleaning
+   * aggresively the memory used, in order to free as much memory as
+   * possible.
+   */
+  MemPressure_New,
+
+  /*
+   * Repeated memory pressure.
+   *
+   * A repeated memory pressure implies to clean softly recent allocations.
+   * It is supposed to happen after a new memory pressure which already
+   * cleaned aggressivley.  So there is no need to damage the reactivity of
+   * Gecko by stopping the world again.
+   *
+   * In case of conflict with an new memory pressue, the new memory pressure
+   * takes precedence over an ongoing memory pressure.  The reason being
+   * that if no events are processed between 2 notifications (new followed
+   * by ongoing, or ongoing followed by a new) we want to be as aggresive as
+   * possible on the clean-up of the memory.  After all, we are trying to
+   * keep Gecko alive as long as possible.
+   */
+  MemPressure_Ongoing
+};
+
+/**
+ * Return and erase the latest state of the memory pressure event set by any of
+ * the corresponding dispatch function.
+ */
+MemoryPressureState
+NS_GetPendingMemoryPressure();
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event, but if there are no events pending in
+ * the main thread's event queue, the memory pressure event would not be
+ * dispatched until one is enqueued. It is infallible and does not allocate
+ * any memory.
+ *
+ * You may call this function from any thread.
+ */
+void
+NS_DispatchEventualMemoryPressure(MemoryPressureState aState);
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event. We wake up the main thread by adding a
+ * dummy event to its event loop, so, unlike with
+ * NS_DispatchEventualMemoryPressure, this memory-pressure event is always
+ * fired relatively quickly, even if the event loop is otherwise empty.
+ *
+ * You may call this function from any thread.
+ */
+nsresult
+NS_DispatchMemoryPressure(MemoryPressureState aState);
+
+#endif // nsMemoryPressure_h__