diff --git a/dom/base/ScriptSettings.cpp b/dom/base/ScriptSettings.cpp
index cb7d097d671b..16ff96c20a64 100644
--- a/dom/base/ScriptSettings.cpp
+++ b/dom/base/ScriptSettings.cpp
@@ -26,7 +26,8 @@
 namespace mozilla {
 namespace dom {
 
-static mozilla::ThreadLocal<ScriptSettingsStackEntry*> sScriptSettingsTLS;
+static MOZ_THREAD_LOCAL(ScriptSettingsStackEntry*) sScriptSettingsTLS;
+static bool sScriptSettingsTLSInitialized;
 
 class ScriptSettingsStack {
 public:
@@ -94,14 +95,13 @@ UnuseEntryScriptProfiling()
 void
 InitScriptSettings()
 {
-  if (!sScriptSettingsTLS.initialized()) {
-    bool success = sScriptSettingsTLS.init();
-    if (!success) {
-      MOZ_CRASH();
-    }
+  bool success = sScriptSettingsTLS.init();
+  if (!success) {
+    MOZ_CRASH();
   }
 
   sScriptSettingsTLS.set(nullptr);
+  sScriptSettingsTLSInitialized = true;
 }
 
 void
@@ -113,7 +113,7 @@ DestroyScriptSettings()
 bool
 ScriptSettingsInitialized()
 {
-  return sScriptSettingsTLS.initialized();
+  return sScriptSettingsTLSInitialized;
 }
 
 ScriptSettingsStackEntry::ScriptSettingsStackEntry(nsIGlobalObject *aGlobal,
diff --git a/dom/promise/PromiseDebugging.cpp b/dom/promise/PromiseDebugging.cpp
index 0f99bb9a003f..6706cd1fd735 100644
--- a/dom/promise/PromiseDebugging.cpp
+++ b/dom/promise/PromiseDebugging.cpp
@@ -60,10 +60,10 @@ public:
 private:
   // `true` if an instance of `FlushRejections` is currently dispatched
   // and has not been executed yet.
-  static ThreadLocal<bool> sDispatched;
+  static MOZ_THREAD_LOCAL(bool) sDispatched;
 };
 
-/* static */ ThreadLocal<bool>
+/* static */ MOZ_THREAD_LOCAL(bool)
 FlushRejections::sDispatched;
 
 static Promise*
diff --git a/gfx/gl/GLLibraryEGL.cpp b/gfx/gl/GLLibraryEGL.cpp
index 3b8370b77506..998141d6dfa2 100644
--- a/gfx/gl/GLLibraryEGL.cpp
+++ b/gfx/gl/GLLibraryEGL.cpp
@@ -29,7 +29,7 @@ namespace gl {
 StaticMutex GLLibraryEGL::sMutex;
 GLLibraryEGL sEGLLibrary;
 #ifdef MOZ_B2G
-ThreadLocal<EGLContext> GLLibraryEGL::sCurrentContext;
+MOZ_THREAD_LOCAL(EGLContext) GLLibraryEGL::sCurrentContext;
 #endif
 
 // should match the order of EGLExtensions, and be null-terminated.
diff --git a/gfx/gl/GLLibraryEGL.h b/gfx/gl/GLLibraryEGL.h
index 81b94de5f9f1..043a8470abcf 100644
--- a/gfx/gl/GLLibraryEGL.h
+++ b/gfx/gl/GLLibraryEGL.h
@@ -641,7 +641,7 @@ public:
     }
 
 private:
-    static ThreadLocal<EGLContext> sCurrentContext;
+    static MOZ_THREAD_LOCAL(EGLContext) sCurrentContext;
 public:
 
 #else
diff --git a/gfx/gl/SkiaGLGlue.cpp b/gfx/gl/SkiaGLGlue.cpp
index 7ffcab1c73ce..9130db42a81f 100755
--- a/gfx/gl/SkiaGLGlue.cpp
+++ b/gfx/gl/SkiaGLGlue.cpp
@@ -24,16 +24,14 @@ using mozilla::gl::GLFeature;
 using mozilla::gl::SkiaGLGlue;
 using mozilla::gfx::DrawTarget;
 
-static mozilla::ThreadLocal<GLContext*> sGLContext;
+static MOZ_THREAD_LOCAL(GLContext*) sGLContext;
 
 extern "C" {
 
 static void SetStaticGLContext(GLContext* context)
 {
-    if (!sGLContext.initialized()) {
-      mozilla::DebugOnly<bool> success = sGLContext.init();
-      MOZ_ASSERT(success);
-    }
+    mozilla::DebugOnly<bool> success = sGLContext.init();
+    MOZ_ASSERT(success);
 
     sGLContext.set(context);
 }
diff --git a/js/src/jit/Ion.cpp b/js/src/jit/Ion.cpp
index ae90f2e64b3a..e93f0237327b 100644
--- a/js/src/jit/Ion.cpp
+++ b/js/src/jit/Ion.cpp
@@ -58,17 +58,15 @@
 using namespace js;
 using namespace js::jit;
 
-using mozilla::ThreadLocal;
-
 // Assert that JitCode is gc::Cell aligned.
 JS_STATIC_ASSERT(sizeof(JitCode) % gc::CellSize == 0);
 
-static ThreadLocal<JitContext*> TlsJitContext;
+static MOZ_THREAD_LOCAL(JitContext*) TlsJitContext;
 
 static JitContext*
 CurrentJitContext()
 {
-    if (!TlsJitContext.initialized())
+    if (!TlsJitContext.init())
         return nullptr;
     return TlsJitContext.get();
 }
@@ -155,7 +153,7 @@ JitContext::~JitContext()
 bool
 jit::InitializeIon()
 {
-    if (!TlsJitContext.initialized() && !TlsJitContext.init())
+    if (!TlsJitContext.init())
         return false;
     CheckLogging();
 #if defined(JS_CODEGEN_ARM)
diff --git a/js/src/jsutil.cpp b/js/src/jsutil.cpp
index 303063fc8565..b9ebfa128fd0 100644
--- a/js/src/jsutil.cpp
+++ b/js/src/jsutil.cpp
@@ -37,11 +37,11 @@ namespace js {
 namespace oom {
 
 JS_PUBLIC_DATA(uint32_t) targetThread = 0;
-JS_PUBLIC_DATA(mozilla::ThreadLocal<uint32_t>) threadType;
+JS_PUBLIC_DATA(MOZ_THREAD_LOCAL(uint32_t)) threadType;
 
 bool
 InitThreadType(void) {
-    return threadType.initialized() || threadType.init();
+    return threadType.init();
 }
 
 void
diff --git a/js/src/vm/Initialization.cpp b/js/src/vm/Initialization.cpp
index 502d640aa008..e5108226621e 100644
--- a/js/src/vm/Initialization.cpp
+++ b/js/src/vm/Initialization.cpp
@@ -75,7 +75,7 @@ JS_Init(void)
 #endif
 
     using js::TlsPerThreadData;
-    if (!TlsPerThreadData.initialized() && !TlsPerThreadData.init())
+    if (!TlsPerThreadData.init())
         return false;
 
 #if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
diff --git a/js/src/vm/Runtime.cpp b/js/src/vm/Runtime.cpp
index 0084df3c9c76..dfc813035e62 100644
--- a/js/src/vm/Runtime.cpp
+++ b/js/src/vm/Runtime.cpp
@@ -63,11 +63,10 @@ using mozilla::NegativeInfinity;
 using mozilla::PodZero;
 using mozilla::PodArrayZero;
 using mozilla::PositiveInfinity;
-using mozilla::ThreadLocal;
 using JS::GenericNaN;
 using JS::DoubleNaNValue;
 
-/* static */ ThreadLocal<PerThreadData*> js::TlsPerThreadData;
+/* static */ MOZ_THREAD_LOCAL(PerThreadData*) js::TlsPerThreadData;
 /* static */ Atomic<size_t> JSRuntime::liveRuntimesCount;
 
 namespace js {
diff --git a/js/src/vm/Runtime.h b/js/src/vm/Runtime.h
index 80df8078fdcf..b35f71320322 100644
--- a/js/src/vm/Runtime.h
+++ b/js/src/vm/Runtime.h
@@ -63,7 +63,7 @@ class TraceLoggerThread;
 #endif
 
 /* Thread Local Storage slot for storing the runtime for a thread. */
-extern mozilla::ThreadLocal<PerThreadData*> TlsPerThreadData;
+extern MOZ_THREAD_LOCAL(PerThreadData*) TlsPerThreadData;
 
 } // namespace js
 
diff --git a/mfbt/ThreadLocal.h b/mfbt/ThreadLocal.h
index eae84a21447c..0ebe2917cb6e 100644
--- a/mfbt/ThreadLocal.h
+++ b/mfbt/ThreadLocal.h
@@ -42,11 +42,28 @@ typedef unsigned long sig_safe_t;
 typedef sig_atomic_t sig_safe_t;
 #endif
 
+namespace detail {
+
+#if defined(HAVE_THREAD_TLS_KEYWORD)
+#define MOZ_HAS_THREAD_LOCAL
+#endif
+
 /*
  * Thread Local Storage helpers.
  *
  * Usage:
  *
+ * Do not directly instantiate this class.  Instead, use the
+ * MOZ_THREAD_LOCAL macro to declare or define instances.  The macro
+ * takes a type name as its argument.
+ *
+ * Declare like this:
+ * extern MOZ_THREAD_LOCAL(int) tlsInt;
+ * Define like this:
+ * MOZ_THREAD_LOCAL(int) tlsInt;
+ * or:
+ * static MOZ_THREAD_LOCAL(int) tlsInt;
+ *
  * Only static-storage-duration (e.g. global variables, or static class members)
  * objects of this class should be instantiated. This class relies on
  * zero-initialization, which is implicit for static-storage-duration objects.
@@ -56,9 +73,10 @@ typedef sig_atomic_t sig_safe_t;
  *
  * // Create a TLS item.
  * //
- * // Note that init() should be invoked exactly once, before any usage of set()
- * // or get().
- * mozilla::ThreadLocal<int> tlsKey;
+ * // Note that init() should be invoked before the first use of set()
+ * // or get().  It is ok to call it multiple times.  This must be
+ * // called in a way that avoids possible races with other threads.
+ * MOZ_THREAD_LOCAL(int) tlsKey;
  * if (!tlsKey.init()) {
  *   // deal with the error
  * }
@@ -72,6 +90,7 @@ typedef sig_atomic_t sig_safe_t;
 template<typename T>
 class ThreadLocal
 {
+#ifndef MOZ_HAS_THREAD_LOCAL
 #if defined(XP_WIN)
   typedef unsigned long key_t;
 #else
@@ -93,11 +112,24 @@ class ThreadLocal
   {
     typedef S *Type;
   };
+#endif
+
+  bool initialized() const {
+#ifdef MOZ_HAS_THREAD_LOCAL
+    return true;
+#else
+    return mInited;
+#endif
+  }
 
 public:
+  // __thread does not allow non-trivial constructors, but we can
+  // instead rely on zero-initialization.
+#ifndef MOZ_HAS_THREAD_LOCAL
   ThreadLocal()
     : mKey(0), mInited(false)
   {}
+#endif
 
   MOZ_WARN_UNUSED_RESULT inline bool init();
 
@@ -105,11 +137,13 @@ public:
 
   inline void set(const T aValue);
 
-  bool initialized() const { return mInited; }
-
 private:
+#ifdef MOZ_HAS_THREAD_LOCAL
+  T mValue;
+#else
   key_t mKey;
   bool mInited;
+#endif
 };
 
 template<typename T>
@@ -122,20 +156,29 @@ ThreadLocal<T>::init()
   static_assert(sizeof(T) <= sizeof(void*),
                 "mozilla::ThreadLocal can't be used for types larger than "
                 "a pointer");
-  MOZ_ASSERT(!initialized());
-#ifdef XP_WIN
-  mKey = TlsAlloc();
-  mInited = mKey != 0xFFFFFFFFUL; // TLS_OUT_OF_INDEXES
+
+#ifdef MOZ_HAS_THREAD_LOCAL
+  return true;
 #else
-  mInited = !pthread_key_create(&mKey, nullptr);
+  if (!initialized()) {
+#ifdef XP_WIN
+    mKey = TlsAlloc();
+    mInited = mKey != 0xFFFFFFFFUL; // TLS_OUT_OF_INDEXES
+#else
+    mInited = !pthread_key_create(&mKey, nullptr);
 #endif
+  }
   return mInited;
+#endif
 }
 
 template<typename T>
 inline T
 ThreadLocal<T>::get() const
 {
+#ifdef MOZ_HAS_THREAD_LOCAL
+  return mValue;
+#else
   MOZ_ASSERT(initialized());
   void* h;
 #ifdef XP_WIN
@@ -144,12 +187,16 @@ ThreadLocal<T>::get() const
   h = pthread_getspecific(mKey);
 #endif
   return static_cast<T>(reinterpret_cast<typename Helper<T>::Type>(h));
+#endif
 }
 
 template<typename T>
 inline void
 ThreadLocal<T>::set(const T aValue)
 {
+#ifdef MOZ_HAS_THREAD_LOCAL
+  mValue = aValue;
+#else
   MOZ_ASSERT(initialized());
   void* h = reinterpret_cast<void*>(static_cast<typename Helper<T>::Type>(aValue));
 #ifdef XP_WIN
@@ -160,8 +207,16 @@ ThreadLocal<T>::set(const T aValue)
   if (!succeeded) {
     MOZ_CRASH();
   }
+#endif
 }
 
+#ifdef MOZ_HAS_THREAD_LOCAL
+#define MOZ_THREAD_LOCAL(TYPE) __thread mozilla::detail::ThreadLocal<TYPE>
+#else
+#define MOZ_THREAD_LOCAL(TYPE) mozilla::detail::ThreadLocal<TYPE>
+#endif
+
+} // namespace detail
 } // namespace mozilla
 
 #endif /* mozilla_ThreadLocal_h */
diff --git a/tools/memory-profiler/UncensoredAllocator.cpp b/tools/memory-profiler/UncensoredAllocator.cpp
index 8d87c709c88e..9cccf44135c7 100644
--- a/tools/memory-profiler/UncensoredAllocator.cpp
+++ b/tools/memory-profiler/UncensoredAllocator.cpp
@@ -20,7 +20,7 @@
 namespace mozilla {
 
 #ifdef MOZ_REPLACE_MALLOC
-ThreadLocal<bool> MallocHook::mEnabledTLS;
+MOZ_THREAD_LOCAL(bool) MallocHook::mEnabledTLS;
 NativeProfiler* MallocHook::mNativeProfiler;
 malloc_hook_table_t MallocHook::mMallocHook;
 #endif
@@ -81,12 +81,10 @@ MallocHook::Initialize()
   if (bridge) {
     mozilla::Unused << bridge->RegisterHook("memory-profiler", nullptr, nullptr);
   }
-  if (!mEnabledTLS.initialized()) {
-    bool success = mEnabledTLS.init();
-    if (NS_WARN_IF(!success)) {
-      return;
-    }
-    mEnabledTLS.set(false);
+
+  bool success = mEnabledTLS.init();
+  if (NS_WARN_IF(!success)) {
+    return;
   }
 #endif
 }
@@ -96,9 +94,6 @@ MallocHook::Enable(NativeProfiler* aNativeProfiler)
 {
 #ifdef MOZ_REPLACE_MALLOC
   MOZ_ASSERT(NS_IsMainThread());
-  if (NS_WARN_IF(!mEnabledTLS.initialized())) {
-    return;
-  }
   ReplaceMallocBridge* bridge = ReplaceMallocBridge::Get(3);
   if (bridge) {
     const malloc_table_t* alloc_funcs =
diff --git a/tools/memory-profiler/UncensoredAllocator.h b/tools/memory-profiler/UncensoredAllocator.h
index c59a05a568cc..b9074c73865e 100644
--- a/tools/memory-profiler/UncensoredAllocator.h
+++ b/tools/memory-profiler/UncensoredAllocator.h
@@ -29,7 +29,7 @@ private:
   static void* SampleNative(void* aAddr, size_t aSize);
   static void RemoveNative(void* aAddr);
 #ifdef MOZ_REPLACE_MALLOC
-  static ThreadLocal<bool> mEnabledTLS;
+  static MOZ_THREAD_LOCAL(bool) mEnabledTLS;
   static NativeProfiler* mNativeProfiler;
   static malloc_hook_table_t mMallocHook;
 #endif
diff --git a/tools/profiler/core/platform.cpp b/tools/profiler/core/platform.cpp
index 7fb57d6ef8cb..3a9726da306e 100644
--- a/tools/profiler/core/platform.cpp
+++ b/tools/profiler/core/platform.cpp
@@ -67,9 +67,9 @@ public:
 };
 #endif
 
-mozilla::ThreadLocal<PseudoStack *> tlsPseudoStack;
-mozilla::ThreadLocal<GeckoSampler *> tlsTicker;
-mozilla::ThreadLocal<void *> tlsStackTop;
+MOZ_THREAD_LOCAL(PseudoStack *) tlsPseudoStack;
+MOZ_THREAD_LOCAL(GeckoSampler *) tlsTicker;
+MOZ_THREAD_LOCAL(void *) tlsStackTop;
 // We need to track whether we've been initialized otherwise
 // we end up using tlsStack without initializing it.
 // Because tlsStack is totally opaque to us we can't reuse
diff --git a/tools/profiler/public/GeckoProfilerImpl.h b/tools/profiler/public/GeckoProfilerImpl.h
index 2b8907f54eda..2b565af15c55 100644
--- a/tools/profiler/public/GeckoProfilerImpl.h
+++ b/tools/profiler/public/GeckoProfilerImpl.h
@@ -40,9 +40,9 @@ namespace mozilla {
 class TimeStamp;
 } // namespace mozilla
 
-extern mozilla::ThreadLocal<PseudoStack *> tlsPseudoStack;
-extern mozilla::ThreadLocal<GeckoSampler *> tlsTicker;
-extern mozilla::ThreadLocal<void *> tlsStackTop;
+extern MOZ_THREAD_LOCAL(PseudoStack *) tlsPseudoStack;
+extern MOZ_THREAD_LOCAL(GeckoSampler *) tlsTicker;
+extern MOZ_THREAD_LOCAL(void *) tlsStackTop;
 extern bool stack_key_initialized;
 
 #ifndef SAMPLE_FUNCTION_NAME
diff --git a/tools/profiler/tasktracer/GeckoTaskTracer.cpp b/tools/profiler/tasktracer/GeckoTaskTracer.cpp
index e1a855c1827f..9895626d3196 100644
--- a/tools/profiler/tasktracer/GeckoTaskTracer.cpp
+++ b/tools/profiler/tasktracer/GeckoTaskTracer.cpp
@@ -59,7 +59,7 @@ pid_t gettid();
 namespace mozilla {
 namespace tasktracer {
 
-static mozilla::ThreadLocal<TraceInfo*> sTraceInfoTLS;
+static MOZ_THREAD_LOCAL(TraceInfo*) sTraceInfoTLS;
 static mozilla::StaticMutex sMutex;
 
 // The generation of TraceInfo. It will be > 0 if the Task Tracer is started and
diff --git a/xpcom/base/nsCycleCollector.cpp b/xpcom/base/nsCycleCollector.cpp
index 16257500debc..82e0b412b319 100644
--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -1409,7 +1409,7 @@ struct CollectorData
   CycleCollectedJSRuntime* mRuntime;
 };
 
-static mozilla::ThreadLocal<CollectorData*> sCollectorData;
+static MOZ_THREAD_LOCAL(CollectorData*) sCollectorData;
 
 ////////////////////////////////////////////////////////////////////////
 // Utility functions
@@ -3999,8 +3999,11 @@ nsCycleCollector_suspectedCount()
 bool
 nsCycleCollector_init()
 {
+  static DebugOnly<bool> sInitialized;
+
   MOZ_ASSERT(NS_IsMainThread(), "Wrong thread!");
-  MOZ_ASSERT(!sCollectorData.initialized(), "Called twice!?");
+  MOZ_ASSERT(!sInitialized, "Called twice!?");
+  sInitialized = true;
 
   return sCollectorData.init();
 }
@@ -4008,8 +4011,6 @@ nsCycleCollector_init()
 void
 nsCycleCollector_startup()
 {
-  MOZ_ASSERT(sCollectorData.initialized(),
-             "Forgot to call nsCycleCollector_init!");
   if (sCollectorData.get()) {
     MOZ_CRASH();
   }
diff --git a/xpcom/build/IOInterposer.cpp b/xpcom/build/IOInterposer.cpp
index 26ad0045aeb5..09e6bdc46821 100644
--- a/xpcom/build/IOInterposer.cpp
+++ b/xpcom/build/IOInterposer.cpp
@@ -359,7 +359,8 @@ public:
 
 // List of observers registered
 static StaticAutoPtr<MasterList> sMasterList;
-static ThreadLocal<PerThreadData*> sThreadLocalData;
+static MOZ_THREAD_LOCAL(PerThreadData*) sThreadLocalData;
+static bool sThreadLocalDataInitialized;
 } // namespace
 
 IOInterposeObserver::Observation::Observation(Operation aOperation,
@@ -429,6 +430,7 @@ IOInterposer::Init()
   if (!sThreadLocalData.init()) {
     return false;
   }
+  sThreadLocalDataInitialized = true;
   bool isMainThread = true;
   RegisterCurrentThread(isMainThread);
   sMasterList = new MasterList();
@@ -448,7 +450,7 @@ IOInterposer::Init()
 bool
 IOInterposeObserver::IsMainThread()
 {
-  if (!sThreadLocalData.initialized()) {
+  if (!sThreadLocalDataInitialized) {
     return false;
   }
   PerThreadData* ptd = sThreadLocalData.get();
@@ -538,7 +540,7 @@ IOInterposer::Unregister(IOInterposeObserver::Operation aOp,
 void
 IOInterposer::RegisterCurrentThread(bool aIsMainThread)
 {
-  if (!sThreadLocalData.initialized()) {
+  if (!sThreadLocalDataInitialized) {
     return;
   }
   MOZ_ASSERT(!sThreadLocalData.get());
@@ -549,7 +551,7 @@ IOInterposer::RegisterCurrentThread(bool aIsMainThread)
 void
 IOInterposer::UnregisterCurrentThread()
 {
-  if (!sThreadLocalData.initialized()) {
+  if (!sThreadLocalDataInitialized) {
     return;
   }
   PerThreadData* curThreadData = sThreadLocalData.get();
diff --git a/xpcom/threads/AbstractThread.cpp b/xpcom/threads/AbstractThread.cpp
index 50eb76ea4622..24a29779d852 100644
--- a/xpcom/threads/AbstractThread.cpp
+++ b/xpcom/threads/AbstractThread.cpp
@@ -26,7 +26,7 @@ LazyLogModule gMozPromiseLog("MozPromise");
 LazyLogModule gStateWatchingLog("StateWatching");
 
 StaticRefPtr<AbstractThread> sMainThread;
-ThreadLocal<AbstractThread*> AbstractThread::sCurrentThreadTLS;
+MOZ_THREAD_LOCAL(AbstractThread*) AbstractThread::sCurrentThreadTLS;
 
 class XPCOMThreadWrapper : public AbstractThread
 {
diff --git a/xpcom/threads/AbstractThread.h b/xpcom/threads/AbstractThread.h
index 0f8d915120e4..6242337c3404 100644
--- a/xpcom/threads/AbstractThread.h
+++ b/xpcom/threads/AbstractThread.h
@@ -85,7 +85,7 @@ public:
 
 protected:
   virtual ~AbstractThread() {}
-  static ThreadLocal<AbstractThread*> sCurrentThreadTLS;
+  static MOZ_THREAD_LOCAL(AbstractThread*) sCurrentThreadTLS;
 
   // True if we want to require that every task dispatched from tasks running in
   // this queue go through our queue's tail dispatcher.
diff --git a/xpcom/threads/BackgroundHangMonitor.cpp b/xpcom/threads/BackgroundHangMonitor.cpp
index 4ea84b0e9763..ba674cbfbe7f 100644
--- a/xpcom/threads/BackgroundHangMonitor.cpp
+++ b/xpcom/threads/BackgroundHangMonitor.cpp
@@ -138,7 +138,8 @@ BackgroundHangManager::Observe(nsISupports* aSubject, const char* aTopic, const
 class BackgroundHangThread : public LinkedListElement<BackgroundHangThread>
 {
 private:
-  static ThreadLocal<BackgroundHangThread*> sTlsKey;
+  static MOZ_THREAD_LOCAL(BackgroundHangThread*) sTlsKey;
+  static bool sTlsKeyInitialized;
 
   BackgroundHangThread(const BackgroundHangThread&);
   BackgroundHangThread& operator=(const BackgroundHangThread&);
@@ -206,8 +207,8 @@ StaticRefPtr<BackgroundHangManager> BackgroundHangManager::sInstance;
 bool BackgroundHangManager::sProhibited = false;
 bool BackgroundHangManager::sDisabled = false;
 
-ThreadLocal<BackgroundHangThread*> BackgroundHangThread::sTlsKey;
-
+MOZ_THREAD_LOCAL(BackgroundHangThread*) BackgroundHangThread::sTlsKey;
+bool BackgroundHangThread::sTlsKeyInitialized;
 
 BackgroundHangManager::BackgroundHangManager()
   : mShutdown(false)
@@ -372,7 +373,7 @@ BackgroundHangThread::BackgroundHangThread(const char* aName,
   , mWaiting(true)
   , mStats(aName)
 {
-  if (sTlsKey.initialized()) {
+  if (sTlsKeyInitialized) {
     sTlsKey.set(this);
   }
   // Lock here because LinkedList is not thread-safe
@@ -393,7 +394,7 @@ BackgroundHangThread::~BackgroundHangThread()
   autoLock.Notify();
 
   // We no longer have a thread
-  if (sTlsKey.initialized()) {
+  if (sTlsKeyInitialized) {
     sTlsKey.set(nullptr);
   }
 
@@ -489,7 +490,7 @@ BackgroundHangThread::FindThread()
     return nullptr;
   }
 
-  if (sTlsKey.initialized()) {
+  if (sTlsKeyInitialized) {
     // Use TLS if available
     MOZ_ASSERT(!BackgroundHangManager::sProhibited,
                "BackgroundHandleManager is not initialized");
diff --git a/xpcom/threads/nsThreadManager.cpp b/xpcom/threads/nsThreadManager.cpp
index c5741d995db2..8eb763d5134d 100644
--- a/xpcom/threads/nsThreadManager.cpp
+++ b/xpcom/threads/nsThreadManager.cpp
@@ -18,7 +18,7 @@
 
 using namespace mozilla;
 
-static mozilla::ThreadLocal<bool> sTLSIsMainThread;
+static MOZ_THREAD_LOCAL(bool) sTLSIsMainThread;
 
 bool
 NS_IsMainThread()
@@ -29,12 +29,10 @@ NS_IsMainThread()
 void
 NS_SetMainThread()
 {
-  if (!sTLSIsMainThread.initialized()) {
-    if (!sTLSIsMainThread.init()) {
-      MOZ_CRASH();
-    }
-    sTLSIsMainThread.set(true);
+  if (!sTLSIsMainThread.init()) {
+    MOZ_CRASH();
   }
+  sTLSIsMainThread.set(true);
   MOZ_ASSERT(NS_IsMainThread());
 }