new file mode 100644
--- /dev/null
+++ b/ipc/mscom/EnsureMTA.cpp
@@ -0,0 +1,78 @@
+/* -*- 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/. */
+
+#include "mozilla/mscom/EnsureMTA.h"
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/StaticPtr.h"
+#include "nsThreadUtils.h"
+
+#include "private/pprthred.h"
+
+namespace {
+
+class EnterMTARunnable : public mozilla::Runnable
+{
+public:
+  NS_IMETHOD Run() override
+  {
+    mozilla::DebugOnly<HRESULT> hr = ::CoInitializeEx(nullptr,
+                                                      COINIT_MULTITHREADED);
+    MOZ_ASSERT(SUCCEEDED(hr));
+    return NS_OK;
+  }
+};
+
+class BackgroundMTAData
+{
+public:
+  BackgroundMTAData()
+  {
+    nsCOMPtr<nsIRunnable> runnable = new EnterMTARunnable();
+    nsresult rv = NS_NewNamedThread("COM MTA",
+                                    getter_AddRefs(mThread), runnable);
+    NS_WARN_IF(NS_FAILED(rv));
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+  }
+
+  ~BackgroundMTAData()
+  {
+    if (mThread) {
+      mThread->Dispatch(NS_NewRunnableFunction(&::CoUninitialize),
+                        NS_DISPATCH_NORMAL);
+      mThread->Shutdown();
+    }
+  }
+
+  nsCOMPtr<nsIThread> GetThread() const
+  {
+    return mThread;
+  }
+
+private:
+  nsCOMPtr<nsIThread> mThread;
+};
+
+} // anonymous namespace
+
+static mozilla::StaticAutoPtr<BackgroundMTAData> sMTAData;
+
+namespace mozilla {
+namespace mscom {
+
+/* static */ nsCOMPtr<nsIThread>
+EnsureMTA::GetMTAThread()
+{
+  if (!sMTAData) {
+    sMTAData = new BackgroundMTAData();
+    ClearOnShutdown(&sMTAData, ShutdownPhase::ShutdownThreads);
+  }
+  return sMTAData->GetThread();
+}
+
+} // namespace mscom
+} // namespace mozilla
+

new file mode 100644
--- /dev/null
+++ b/ipc/mscom/EnsureMTA.h
@@ -0,0 +1,77 @@
+/* -*- 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/Function.h"
+#include "mozilla/mscom/COMApartmentRegion.h"
+#include "mozilla/mscom/utils.h"
+#include "nsCOMPtr.h"
+#include "nsIThread.h"
+#include "nsThreadUtils.h"
+
+#include <windows.h>
+
+namespace mozilla {
+namespace mscom {
+
+// This class is OK to use as a temporary on the stack.
+class MOZ_STACK_CLASS EnsureMTA
+{
+public:
+  template <typename FuncT>
+  EnsureMTA(const FuncT& aClosure)
+  {
+    MOZ_ASSERT(NS_IsMainThread());
+    if (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);
+
+    HANDLE event = ::CreateEventW(nullptr, FALSE, FALSE, nullptr);
+    if (!event) {
+      return;
+    }
+
+    auto eventSetter = [&]() -> void {
+      aClosure();
+      ::SetEvent(event);
+    };
+
+    nsresult rv =
+      thread->Dispatch(NS_NewRunnableFunction(eventSetter), NS_DISPATCH_NORMAL);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      ::CloseHandle(event);
+      return;
+    }
+
+    DWORD waitResult;
+    while ((waitResult = ::WaitForSingleObjectEx(event, INFINITE, TRUE)) ==
+           WAIT_IO_COMPLETION) {
+    }
+    MOZ_ASSERT(waitResult == WAIT_OBJECT_0);
+    ::CloseHandle(event);
+  }
+
+private:
+  static nsCOMPtr<nsIThread> GetMTAThread();
+};
+
+} // namespace mscom
+} // namespace mozilla
+
+#endif // mozilla_mscom_EnsureMTA_h
+

--- a/ipc/mscom/moz.build
+++ b/ipc/mscom/moz.build
@@ -1,23 +1,25 @@
 # -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
 # vim: set filetype=python:
 # 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/.
 
 EXPORTS.mozilla.mscom += [
     'COMApartmentRegion.h',
+    'EnsureMTA.h',
     'MainThreadRuntime.h',
     'Utils.h',
 ]
 
 SOURCES += [
     'Utils.cpp',
 ]
 
 UNIFIED_SOURCES += [
+    'EnsureMTA.cpp',
     'MainThreadRuntime.cpp',
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'