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diff --git a/xpcom/threads/Mutex.h b/xpcom/threads/Mutex.h
new file mode 100644
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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_Mutex_h
+#define mozilla_Mutex_h
+
+#include "mozilla/BlockingResourceBase.h"
+#include "mozilla/PlatformMutex.h"
+#include "nsISupports.h"
+
+//
+// Provides:
+//
+//  - Mutex, a non-recursive mutex
+//  - MutexAutoLock, an RAII class for ensuring that Mutexes are properly
+//    locked and unlocked
+//  - MutexAutoUnlock, complementary sibling to MutexAutoLock
+//
+//  - OffTheBooksMutex, a non-recursive mutex that doesn't do leak checking
+//  - OffTheBooksMutexAuto{Lock,Unlock} - Like MutexAuto{Lock,Unlock}, but for
+//    an OffTheBooksMutex.
+//
+// Using MutexAutoLock/MutexAutoUnlock etc. is MUCH preferred to making bare
+// calls to Lock and Unlock.
+//
+namespace mozilla {
+
+/**
+ * OffTheBooksMutex is identical to Mutex, except that OffTheBooksMutex doesn't
+ * include leak checking.  Sometimes you want to intentionally "leak" a mutex
+ * until shutdown; in these cases, OffTheBooksMutex is for you.
+ */
+class OffTheBooksMutex : public detail::MutexImpl, BlockingResourceBase {
+ public:
+  /**
+   * @param aName A name which can reference this lock
+   * @returns If failure, nullptr
+   *          If success, a valid Mutex* which must be destroyed
+   *          by Mutex::DestroyMutex()
+   **/
+  explicit OffTheBooksMutex(const char* aName)
+      : BlockingResourceBase(aName, eMutex)
+#ifdef DEBUG
+        ,
+        mOwningThread(nullptr)
+#endif
+  {
+  }
+
+  ~OffTheBooksMutex() {
+#ifdef DEBUG
+    MOZ_ASSERT(!mOwningThread, "destroying a still-owned lock!");
+#endif
+  }
+
+#ifndef DEBUG
+  /**
+   * Lock this mutex.
+   **/
+  void Lock() { this->lock(); }
+
+  /**
+   * Try to lock this mutex, returning true if we were successful.
+   **/
+  [[nodiscard]] bool TryLock() { return this->tryLock(); }
+
+  /**
+   * Unlock this mutex.
+   **/
+  void Unlock() { this->unlock(); }
+
+  /**
+   * Assert that the current thread owns this mutex in debug builds.
+   *
+   * Does nothing in non-debug builds.
+   **/
+  void AssertCurrentThreadOwns() const {}
+
+  /**
+   * Assert that the current thread does not own this mutex.
+   *
+   * Note that this function is not implemented for debug builds *and*
+   * non-debug builds due to difficulties in dealing with memory ordering.
+   *
+   * It is therefore mostly useful as documentation.
+   **/
+  void AssertNotCurrentThreadOwns() const {}
+
+#else
+  void Lock();
+  [[nodiscard]] bool TryLock();
+  void Unlock();
+
+  void AssertCurrentThreadOwns() const;
+
+  void AssertNotCurrentThreadOwns() const {
+    // FIXME bug 476536
+  }
+
+#endif  // ifndef DEBUG
+
+ private:
+  OffTheBooksMutex();
+  OffTheBooksMutex(const OffTheBooksMutex&);
+  OffTheBooksMutex& operator=(const OffTheBooksMutex&);
+
+  friend class OffTheBooksCondVar;
+
+#ifdef DEBUG
+  PRThread* mOwningThread;
+#endif
+};
+
+/**
+ * Mutex
+ * When possible, use MutexAutoLock/MutexAutoUnlock to lock/unlock this
+ * mutex within a scope, instead of calling Lock/Unlock directly.
+ */
+class Mutex : public OffTheBooksMutex {
+ public:
+  explicit Mutex(const char* aName) : OffTheBooksMutex(aName) {
+    MOZ_COUNT_CTOR(Mutex);
+  }
+
+  MOZ_COUNTED_DTOR(Mutex)
+
+ private:
+  Mutex();
+  Mutex(const Mutex&);
+  Mutex& operator=(const Mutex&);
+};
+
+namespace detail {
+template <typename T>
+class MOZ_RAII BaseAutoUnlock;
+
+/**
+ * MutexAutoLock
+ * Acquires the Mutex when it enters scope, and releases it when it leaves
+ * scope.
+ *
+ * MUCH PREFERRED to bare calls to Mutex.Lock and Unlock.
+ */
+template <typename T>
+class MOZ_RAII BaseAutoLock {
+ public:
+  /**
+   * Constructor
+   * The constructor aquires the given lock.  The destructor
+   * releases the lock.
+   *
+   * @param aLock A valid mozilla::Mutex* returned by
+   *              mozilla::Mutex::NewMutex.
+   **/
+  explicit BaseAutoLock(T aLock) : mLock(aLock) { mLock.Lock(); }
+
+  ~BaseAutoLock(void) { mLock.Unlock(); }
+
+  // Assert that aLock is the mutex passed to the constructor and that the
+  // current thread owns the mutex.  In coding patterns such as:
+  //
+  // void LockedMethod(const BaseAutoLock<T>& aProofOfLock)
+  // {
+  //   aProofOfLock.AssertOwns(mMutex);
+  //   ...
+  // }
+  //
+  // Without this assertion, it could be that mMutex is not actually
+  // locked. It's possible to have code like:
+  //
+  // BaseAutoLock lock(someMutex);
+  // ...
+  // BaseAutoUnlock unlock(someMutex);
+  // ...
+  // LockedMethod(lock);
+  //
+  // and in such a case, simply asserting that the mutex pointers match is not
+  // sufficient; mutex ownership must be asserted as well.
+  //
+  // Note that if you are going to use the coding pattern presented above, you
+  // should use this method in preference to using AssertCurrentThreadOwns on
+  // the mutex you expected to be held, since this method provides stronger
+  // guarantees.
+  void AssertOwns(const T& aMutex) const {
+    MOZ_ASSERT(&aMutex == &mLock);
+    mLock.AssertCurrentThreadOwns();
+  }
+
+ private:
+  BaseAutoLock();
+  BaseAutoLock(BaseAutoLock&);
+  BaseAutoLock& operator=(BaseAutoLock&);
+  static void* operator new(size_t) noexcept(true);
+
+  friend class BaseAutoUnlock<T>;
+
+  T mLock;
+};
+
+template <typename MutexType>
+BaseAutoLock(MutexType&) -> BaseAutoLock<MutexType&>;
+}  // namespace detail
+
+typedef detail::BaseAutoLock<Mutex&> MutexAutoLock;
+typedef detail::BaseAutoLock<OffTheBooksMutex&> OffTheBooksMutexAutoLock;
+
+namespace detail {
+/**
+ * BaseAutoUnlock
+ * Releases the Mutex when it enters scope, and re-acquires it when it leaves
+ * scope.
+ *
+ * MUCH PREFERRED to bare calls to Mutex.Unlock and Lock.
+ */
+template <typename T>
+class MOZ_RAII BaseAutoUnlock {
+ public:
+  explicit BaseAutoUnlock(T aLock) : mLock(aLock) { mLock.Unlock(); }
+
+  explicit BaseAutoUnlock(BaseAutoLock<T>& aAutoLock) : mLock(aAutoLock.mLock) {
+    NS_ASSERTION(mLock, "null lock");
+    mLock->Unlock();
+  }
+
+  ~BaseAutoUnlock() { mLock.Lock(); }
+
+ private:
+  BaseAutoUnlock();
+  BaseAutoUnlock(BaseAutoUnlock&);
+  BaseAutoUnlock& operator=(BaseAutoUnlock&);
+  static void* operator new(size_t) noexcept(true);
+
+  T mLock;
+};
+
+template <typename MutexType>
+BaseAutoUnlock(MutexType&) -> BaseAutoUnlock<MutexType&>;
+}  // namespace detail
+
+typedef detail::BaseAutoUnlock<Mutex&> MutexAutoUnlock;
+typedef detail::BaseAutoUnlock<OffTheBooksMutex&> OffTheBooksMutexAutoUnlock;
+
+}  // namespace mozilla
+
+#endif  // ifndef mozilla_Mutex_h