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diff --git a/ipc/mscom/EnsureMTA.h b/ipc/mscom/EnsureMTA.h
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+++ b/ipc/mscom/EnsureMTA.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 mozilla_mscom_EnsureMTA_h
+#define mozilla_mscom_EnsureMTA_h
+
+#include "MainThreadUtils.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/Unused.h"
+#include "mozilla/mscom/AgileReference.h"
+#include "mozilla/mscom/Utils.h"
+#include "mozilla/RefPtr.h"
+#include "nsCOMPtr.h"
+#include "nsIThread.h"
+#include "nsThreadUtils.h"
+#include "nsWindowsHelpers.h"
+
+#include <windows.h>
+
+namespace mozilla {
+namespace mscom {
+namespace detail {
+
+// Forward declarations
+template <typename T>
+struct MTADelete;
+
+template <typename T>
+struct MTARelease;
+
+template <typename T>
+struct MTAReleaseInChildProcess;
+
+struct PreservedStreamDeleter;
+
+}  // namespace detail
+
+// This class is OK to use as a temporary on the stack.
+class MOZ_STACK_CLASS EnsureMTA final {
+ public:
+  /**
+   * This constructor just ensures that the MTA thread is up and running.
+   */
+  EnsureMTA() {
+    nsCOMPtr<nsIThread> thread = GetMTAThread();
+    MOZ_ASSERT(thread);
+    Unused << thread;
+  }
+
+  enum class Option {
+    Default,
+    // Forcibly dispatch to the thread returned by GetMTAThread(), even if the
+    // current thread is already inside a MTA.
+    ForceDispatch,
+  };
+
+  /**
+   * Synchronously run |aClosure| on a thread living in the COM multithreaded
+   * apartment. If the current thread lives inside the COM MTA, then it runs
+   * |aClosure| immediately unless |aOpt| == Option::ForceDispatch.
+   */
+  template <typename FuncT>
+  explicit EnsureMTA(FuncT&& aClosure, Option aOpt = Option::Default) {
+    if (aOpt != Option::ForceDispatch && IsCurrentThreadMTA()) {
+      // We're already on the MTA, we can run aClosure directly
+      aClosure();
+      return;
+    }
+
+    // In this case we need to run aClosure on a background thread in the MTA
+    nsCOMPtr<nsIThread> thread = GetMTAThread();
+    MOZ_ASSERT(thread);
+    if (!thread) {
+      return;
+    }
+
+    // Note that we might reenter the EnsureMTA constructor while we wait on
+    // this event due to APC dispatch, therefore we need a unique event object
+    // for each entry. If perf becomes an issue then we will want to maintain
+    // an array of events where the Nth event is unique to the Nth reentry.
+    nsAutoHandle event(::CreateEventW(nullptr, FALSE, FALSE, nullptr));
+    if (!event) {
+      return;
+    }
+
+    HANDLE eventHandle = event.get();
+
+    auto eventSetter = [&aClosure, eventHandle]() -> void {
+      aClosure();
+      ::SetEvent(eventHandle);
+    };
+
+    nsresult rv = thread->Dispatch(
+        NS_NewRunnableFunction("EnsureMTA", std::move(eventSetter)),
+        NS_DISPATCH_NORMAL);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+    if (NS_FAILED(rv)) {
+      return;
+    }
+
+    DWORD waitResult;
+    while ((waitResult = ::WaitForSingleObjectEx(event, INFINITE, TRUE)) ==
+           WAIT_IO_COMPLETION) {
+    }
+    MOZ_ASSERT(waitResult == WAIT_OBJECT_0);
+  }
+
+  using CreateInstanceAgileRefPromise =
+      MozPromise<AgileReference, HRESULT, false>;
+
+  /**
+   *       *** A MSCOM PEER SHOULD REVIEW ALL NEW USES OF THIS API! ***
+   *
+   * Asynchronously instantiate a new COM object from a MTA thread, unless the
+   * current thread is already living inside the multithreaded apartment, in
+   * which case the object is immediately instantiated.
+   *
+   * This function only supports the most common configurations for creating
+   * a new object, so it only supports in-process servers. Furthermore, this
+   * function does not support aggregation (ie. the |pUnkOuter| parameter to
+   * CoCreateInstance).
+   *
+   * Given that attempting to instantiate an Apartment-threaded COM object
+   * inside the MTA results in a *loss* of performance, we assert when that
+   * situation arises.
+   *
+   * The resulting promise, once resolved, provides an AgileReference that may
+   * be passed between any COM-initialized thread in the current process.
+   *
+   *       *** A MSCOM PEER SHOULD REVIEW ALL NEW USES OF THIS API! ***
+   *
+   * WARNING:
+   * Some COM objects do not support creation in the multithreaded apartment,
+   * in which case this function is not available as an option. In this case,
+   * the promise will always be rejected. In debug builds we will assert.
+   *
+   *       *** A MSCOM PEER SHOULD REVIEW ALL NEW USES OF THIS API! ***
+   *
+   * WARNING:
+   * Any in-process COM objects whose interfaces accept HWNDs are probably
+   * *not* safe to instantiate in the multithreaded apartment! Even if this
+   * function succeeds when creating such an object, you *MUST NOT* do so, as
+   * these failures might not become apparent until your code is running out in
+   * the wild on the release channel!
+   *
+   *       *** A MSCOM PEER SHOULD REVIEW ALL NEW USES OF THIS API! ***
+   *
+   * WARNING:
+   * When you obtain an interface from the AgileReference, it may or may not be
+   * a proxy to the real object. This depends entirely on the implementation of
+   * the underlying class and the multithreading capabilities that the class
+   * declares to the COM runtime. If the interface is proxied, it might be
+   * expensive to invoke methods on that interface! *Always* test the
+   * performance of your method calls when calling interfaces that are resolved
+   * via this function!
+   *
+   *       *** A MSCOM PEER SHOULD REVIEW ALL NEW USES OF THIS API! ***
+   *
+   * (Despite this myriad of warnings, it is still *much* safer to use this
+   * function to asynchronously create COM objects than it is to roll your own!)
+   *
+   *       *** A MSCOM PEER SHOULD REVIEW ALL NEW USES OF THIS API! ***
+   */
+  static RefPtr<CreateInstanceAgileRefPromise> CreateInstance(REFCLSID aClsid,
+                                                              REFIID aIid);
+
+ private:
+  static RefPtr<CreateInstanceAgileRefPromise> CreateInstanceInternal(
+      REFCLSID aClsid, REFIID aIid);
+
+  static nsCOMPtr<nsIThread> GetMTAThread();
+
+  // The following function is private in order to force any consumers to be
+  // declared as friends of EnsureMTA. The intention is to prevent
+  // AsyncOperation from becoming some kind of free-for-all mechanism for
+  // asynchronously executing work on a background thread.
+  template <typename FuncT>
+  static void AsyncOperation(FuncT&& aClosure) {
+    if (IsCurrentThreadMTA()) {
+      aClosure();
+      return;
+    }
+
+    nsCOMPtr<nsIThread> thread(GetMTAThread());
+    MOZ_ASSERT(thread);
+    if (!thread) {
+      return;
+    }
+
+    DebugOnly<nsresult> rv = thread->Dispatch(
+        NS_NewRunnableFunction("mscom::EnsureMTA::AsyncOperation",
+                               std::move(aClosure)),
+        NS_DISPATCH_NORMAL);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+  }
+
+  template <typename T>
+  friend struct mozilla::mscom::detail::MTADelete;
+
+  template <typename T>
+  friend struct mozilla::mscom::detail::MTARelease;
+
+  template <typename T>
+  friend struct mozilla::mscom::detail::MTAReleaseInChildProcess;
+
+  friend struct mozilla::mscom::detail::PreservedStreamDeleter;
+};
+
+}  // namespace mscom
+}  // namespace mozilla
+
+#endif  // mozilla_mscom_EnsureMTA_h