diff --git a/dom/storage/StorageDBThread.cpp b/dom/storage/StorageDBThread.cpp
index af605480590c..dc42e2ab1de7 100644
--- a/dom/storage/StorageDBThread.cpp
+++ b/dom/storage/StorageDBThread.cpp
@@ -25,12 +25,8 @@
 #include "mozilla/BasePrincipal.h"
 #include "mozilla/ipc/BackgroundParent.h"
 #include "nsIObserverService.h"
-#include "nsThread.h"
-#include "nsThreadManager.h"
 #include "nsVariant.h"
-#include "mozilla/EventQueue.h"
 #include "mozilla/IOInterposer.h"
-#include "mozilla/ThreadEventQueue.h"
 #include "mozilla/Services.h"
 #include "mozilla/Tokenizer.h"
 #include "GeckoProfiler.h"
@@ -492,12 +488,6 @@ StorageDBThread::SetDefaultPriority()
 void
 StorageDBThread::ThreadFunc(void* aArg)
 {
-  {
-    auto queue = MakeRefPtr<ThreadEventQueue<EventQueue>>(MakeUnique<EventQueue>());
-    Unused <<
-      nsThreadManager::get().CreateCurrentThread(queue, nsThread::NOT_MAIN_THREAD);
-  }
-
   AUTO_PROFILER_REGISTER_THREAD("localStorage DB");
   NS_SetCurrentThreadName("localStorage DB");
   mozilla::IOInterposer::RegisterCurrentThread();
diff --git a/dom/workers/WorkerPrivate.cpp b/dom/workers/WorkerPrivate.cpp
index 46663e576f7c..4855312b8855 100644
--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -5354,13 +5354,22 @@ WorkerPrivate::CreateDebuggerGlobalScope(JSContext* aCx)
 bool
 WorkerPrivate::IsOnWorkerThread() const
 {
-  // We can't use mThread because it must be protected by mMutex and sometimes
-  // this method is called when mMutex is already locked. This method should
-  // always work.
+  // This is much more complicated than it needs to be but we can't use mThread
+  // because it must be protected by mMutex and sometimes this method is called
+  // when mMutex is already locked. This method should always work.
   MOZ_ASSERT(mPRThread,
              "AssertIsOnWorkerThread() called before a thread was assigned!");
 
-  return mPRThread == PR_GetCurrentThread();
+  nsCOMPtr<nsIThread> thread;
+  nsresult rv =
+    nsThreadManager::get().GetThreadFromPRThread(mPRThread,
+                                                 getter_AddRefs(thread));
+  MOZ_ASSERT(NS_SUCCEEDED(rv));
+  MOZ_ASSERT(thread);
+
+  bool current;
+  rv = thread->IsOnCurrentThread(&current);
+  return NS_SUCCEEDED(rv) && current;
 }
 
 #ifdef DEBUG
diff --git a/ipc/chromium/src/base/platform_thread_posix.cc b/ipc/chromium/src/base/platform_thread_posix.cc
index 3984778a315c..1c6410453fcf 100644
--- a/ipc/chromium/src/base/platform_thread_posix.cc
+++ b/ipc/chromium/src/base/platform_thread_posix.cc
@@ -35,6 +35,10 @@ void InitThreading();
 #endif
 
 static void* ThreadFunc(void* closure) {
+  // Create a nsThread wrapper for the current platform thread, and register it
+  // with the thread manager.
+  (void) NS_GetCurrentThread();
+
   PlatformThread::Delegate* delegate =
       static_cast<PlatformThread::Delegate*>(closure);
   delegate->ThreadMain();
diff --git a/ipc/chromium/src/base/platform_thread_win.cc b/ipc/chromium/src/base/platform_thread_win.cc
index e9e84751b2a8..762aa7e305e3 100644
--- a/ipc/chromium/src/base/platform_thread_win.cc
+++ b/ipc/chromium/src/base/platform_thread_win.cc
@@ -25,6 +25,10 @@ typedef struct tagTHREADNAME_INFO {
 } THREADNAME_INFO;
 
 DWORD __stdcall ThreadFunc(void* closure) {
+  // Create a nsThread wrapper for the current platform thread, and register it
+  // with the thread manager.
+  (void) NS_GetCurrentThread();
+
   PlatformThread::Delegate* delegate =
       static_cast<PlatformThread::Delegate*>(closure);
   delegate->ThreadMain();
diff --git a/ipc/chromium/src/base/thread.cc b/ipc/chromium/src/base/thread.cc
index 5a67140a7bf5..79519ea47ab7 100644
--- a/ipc/chromium/src/base/thread.cc
+++ b/ipc/chromium/src/base/thread.cc
@@ -10,11 +10,8 @@
 #include "base/thread_local.h"
 #include "base/waitable_event.h"
 #include "GeckoProfiler.h"
-#include "mozilla/EventQueue.h"
 #include "mozilla/IOInterposer.h"
-#include "mozilla/ThreadEventQueue.h"
 #include "nsThreadUtils.h"
-#include "nsThreadManager.h"
 
 #ifdef MOZ_TASK_TRACER
 #include "GeckoTaskTracer.h"
@@ -157,24 +154,12 @@ void Thread::StopSoon() {
 }
 
 void Thread::ThreadMain() {
-  nsCOMPtr<nsIThread> xpcomThread;
-  if (startup_data_->options.message_loop_type == MessageLoop::TYPE_MOZILLA_NONMAINTHREAD) {
-    auto queue = mozilla::MakeRefPtr<mozilla::ThreadEventQueue<mozilla::EventQueue>>(
-      mozilla::MakeUnique<mozilla::EventQueue>());
-    xpcomThread =
-      nsThreadManager::get().CreateCurrentThread(queue, nsThread::NOT_MAIN_THREAD);
-  } else {
-    xpcomThread = NS_GetCurrentThread();
-  }
-
   AUTO_PROFILER_REGISTER_THREAD(name_.c_str());
   mozilla::IOInterposer::RegisterCurrentThread();
 
   // The message loop for this thread.
   MessageLoop message_loop(startup_data_->options.message_loop_type,
-                           xpcomThread);
-
-  xpcomThread = nullptr;
+                           NS_GetCurrentThread());
 
   // Complete the initialization of our Thread object.
   thread_id_ = PlatformThread::CurrentId();
diff --git a/netwerk/cache2/CacheIOThread.cpp b/netwerk/cache2/CacheIOThread.cpp
index f03e61885736..8a2b0d868506 100644
--- a/netwerk/cache2/CacheIOThread.cpp
+++ b/netwerk/cache2/CacheIOThread.cpp
@@ -8,12 +8,8 @@
 #include "nsIRunnable.h"
 #include "nsISupportsImpl.h"
 #include "nsPrintfCString.h"
-#include "nsThread.h"
-#include "nsThreadManager.h"
 #include "nsThreadUtils.h"
-#include "mozilla/EventQueue.h"
 #include "mozilla/IOInterposer.h"
-#include "mozilla/ThreadEventQueue.h"
 #include "GeckoProfiler.h"
 
 #ifdef XP_WIN
@@ -465,10 +461,8 @@ void CacheIOThread::ThreadFunc()
     MOZ_ASSERT(mBlockingIOWatcher);
     mBlockingIOWatcher->InitThread();
 
-    auto queue = MakeRefPtr<ThreadEventQueue<mozilla::EventQueue>>(
-      MakeUnique<mozilla::EventQueue>());
-    nsCOMPtr<nsIThread> xpcomThread =
-      nsThreadManager::get().CreateCurrentThread(queue, nsThread::NOT_MAIN_THREAD);
+    // This creates nsThread for this PRThread
+    nsCOMPtr<nsIThread> xpcomThread = NS_GetCurrentThread();
 
     threadInternal = do_QueryInterface(xpcomThread);
     if (threadInternal)
diff --git a/security/sandbox/linux/broker/SandboxBroker.cpp b/security/sandbox/linux/broker/SandboxBroker.cpp
index 73e62ff1450f..3a55c98ef52d 100644
--- a/security/sandbox/linux/broker/SandboxBroker.cpp
+++ b/security/sandbox/linux/broker/SandboxBroker.cpp
@@ -31,7 +31,6 @@
 #include "mozilla/ipc/FileDescriptor.h"
 #include "nsDirectoryServiceDefs.h"
 #include "nsAppDirectoryServiceDefs.h"
-#include "nsThreadUtils.h"
 #include "SpecialSystemDirectory.h"
 #include "sandbox/linux/system_headers/linux_syscalls.h"
 
@@ -664,10 +663,6 @@ SandboxBroker::SymlinkPermissions(const char* aPath, const size_t aPathLen)
 void
 SandboxBroker::ThreadMain(void)
 {
-  // Create a nsThread wrapper for the current platform thread, and register it
-  // with the thread manager.
-  (void) NS_GetCurrentThread();
-
   char threadName[16];
   SprintfLiteral(threadName, "FS Broker %d", mChildPid);
   PlatformThread::SetName(threadName);
diff --git a/security/sandbox/linux/reporter/SandboxReporter.cpp b/security/sandbox/linux/reporter/SandboxReporter.cpp
index 86db888648bc..6403c0500c90 100644
--- a/security/sandbox/linux/reporter/SandboxReporter.cpp
+++ b/security/sandbox/linux/reporter/SandboxReporter.cpp
@@ -237,10 +237,6 @@ SandboxReporter::AddOne(const SandboxReport& aReport)
 void
 SandboxReporter::ThreadMain(void)
 {
-  // Create a nsThread wrapper for the current platform thread, and register it
-  // with the thread manager.
-  (void) NS_GetCurrentThread();
-
   for (;;) {
     SandboxReport rep;
     struct iovec iov;
diff --git a/storage/test/gtest/storage_test_harness.h b/storage/test/gtest/storage_test_harness.h
index 31447e1f99b8..09a01c7f9da5 100644
--- a/storage/test/gtest/storage_test_harness.h
+++ b/storage/test/gtest/storage_test_harness.h
@@ -225,7 +225,7 @@ PRThread *watched_thread = nullptr;
  * When the thread a mutex is invoked on isn't watched_thread we save it to this
  * variable.
  */
-nsIThread* last_non_watched_thread = nullptr;
+PRThread *last_non_watched_thread = nullptr;
 
 /**
  * Set a flag if the mutex is used on the thread we are watching, but always
@@ -233,10 +233,11 @@ nsIThread* last_non_watched_thread = nullptr;
  */
 extern "C" void wrapped_MutexEnter(sqlite3_mutex *mutex)
 {
-  if (PR_GetCurrentThread() == watched_thread)
+  PRThread *curThread = ::PR_GetCurrentThread();
+  if (curThread == watched_thread)
     mutex_used_on_watched_thread = true;
   else
-    last_non_watched_thread = NS_GetCurrentThread();
+    last_non_watched_thread = curThread;
   orig_mutex_methods.xMutexEnter(mutex);
 }
 
@@ -350,13 +351,19 @@ get_conn_async_thread(mozIStorageConnection *db)
   blocking_async_execute(stmt);
   stmt->Finalize();
 
-  nsCOMPtr<nsIThread> asyncThread = last_non_watched_thread;
+  nsCOMPtr<nsIThreadManager> threadMan =
+    do_GetService("@mozilla.org/thread-manager;1");
+  nsCOMPtr<nsIThread> asyncThread;
+  threadMan->GetThreadFromPRThread(last_non_watched_thread,
+                                   getter_AddRefs(asyncThread));
 
   // Additionally, check that the thread we get as the background thread is the
   // same one as the one we report from getInterface.
   nsCOMPtr<nsIEventTarget> target = do_GetInterface(db);
   nsCOMPtr<nsIThread> allegedAsyncThread = do_QueryInterface(target);
-  do_check_eq(allegedAsyncThread, asyncThread);
+  PRThread *allegedPRThread;
+  (void)allegedAsyncThread->GetPRThread(&allegedPRThread);
+  do_check_eq(allegedPRThread, last_non_watched_thread);
   return asyncThread.forget();
 }
 
diff --git a/xpcom/threads/nsIThreadManager.idl b/xpcom/threads/nsIThreadManager.idl
index bd0917204748..89d67081f822 100644
--- a/xpcom/threads/nsIThreadManager.idl
+++ b/xpcom/threads/nsIThreadManager.idl
@@ -84,6 +84,19 @@ interface nsIThreadManager : nsISupports
    */
   [noscript] nsIThread newNamedThread(in ACString name, [optional] in unsigned long stackSize);
 
+  /**
+   * Get the nsIThread object (if any) corresponding to the given PRThread.
+   * This method returns null if there is no corresponding nsIThread.
+   *
+   * @param prthread
+   *   The PRThread of the nsIThread being requested.
+   *
+   * @returns
+   *   The nsIThread object corresponding to the given PRThread or null if no
+   *   such nsIThread exists.
+   */
+  [noscript] nsIThread getThreadFromPRThread(in PRThread prthread);
+
   /**
    * Get the main thread.
    */
diff --git a/xpcom/threads/nsThread.cpp b/xpcom/threads/nsThread.cpp
index a4eb95283726..a023114176e1 100644
--- a/xpcom/threads/nsThread.cpp
+++ b/xpcom/threads/nsThread.cpp
@@ -115,9 +115,6 @@ NS_DECL_CI_INTERFACE_GETTER(nsThread)
 
 Array<char, nsThread::kRunnableNameBufSize> nsThread::sMainThreadRunnableName;
 
-uint32_t nsThread::sActiveThreads;
-uint32_t nsThread::sMaxActiveThreads;
-
 //-----------------------------------------------------------------------------
 // Because we do not have our own nsIFactory, we have to implement nsIClassInfo
 // somewhat manually.
@@ -419,40 +416,6 @@ nsThread::Enumerate()
   return {};
 }
 
-/* static */ uint32_t
-nsThread::MaxActiveThreads()
-{
-  OffTheBooksMutexAutoLock mal(ThreadListMutex());
-  return sMaxActiveThreads;
-}
-
-void
-nsThread::AddToThreadList()
-{
-  OffTheBooksMutexAutoLock mal(ThreadListMutex());
-  MOZ_ASSERT(!isInList());
-
-  sActiveThreads++;
-  sMaxActiveThreads = std::max(sActiveThreads, sMaxActiveThreads);
-
-  ThreadList().insertBack(this);
-}
-
-void
-nsThread::MaybeRemoveFromThreadList()
-{
-  // We shouldn't need to lock before checking isInList at this point. We're
-  // destroying the last reference to this object, so there's no way for anyone
-  // else to remove it in the middle of our check. And the not-in-list state is
-  // determined the element's next and previous members pointing to itself, so a
-  // non-atomic update to an adjacent member won't affect the outcome either.
-  if (isInList()) {
-    OffTheBooksMutexAutoLock mal(ThreadListMutex());
-    sActiveThreads--;
-    removeFrom(ThreadList());
-  }
-}
-
 /*static*/ void
 nsThread::ThreadFunc(void* aArg)
 {
@@ -461,9 +424,6 @@ nsThread::ThreadFunc(void* aArg)
   ThreadInitData* initData = static_cast<ThreadInitData*>(aArg);
   nsThread* self = initData->thread;  // strong reference
 
-  MOZ_ASSERT(self->mEventTarget);
-  MOZ_ASSERT(self->mEvents);
-
   self->mThread = PR_GetCurrentThread();
   self->mVirtualThread = GetCurrentVirtualThread();
   self->mEventTarget->SetCurrentThread();
@@ -625,7 +585,8 @@ nsThread::InitCommon()
 #endif
   }
 
-  AddToThreadList();
+  OffTheBooksMutexAutoLock mal(ThreadListMutex());
+  ThreadList().insertBack(this);
 }
 
 //-----------------------------------------------------------------------------
@@ -655,33 +616,20 @@ nsThread::nsThread(NotNull<SynchronizedEventQueue*> aQueue,
 {
 }
 
-
-nsThread::nsThread()
-  : mEvents(nullptr)
-  , mEventTarget(nullptr)
-  , mShutdownContext(nullptr)
-  , mScriptObserver(nullptr)
-  , mThread(nullptr)
-  , mStackSize(0)
-  , mNestedEventLoopDepth(0)
-  , mCurrentEventLoopDepth(-1)
-  , mShutdownRequired(false)
-  , mPriority(PRIORITY_NORMAL)
-  , mIsMainThread(NOT_MAIN_THREAD)
-  , mCanInvokeJS(false)
-  , mCurrentEvent(nullptr)
-  , mCurrentEventStart(TimeStamp::Now())
-  , mCurrentPerformanceCounter(nullptr)
-{
-  MOZ_ASSERT(!NS_IsMainThread());
-}
-
 nsThread::~nsThread()
 {
   NS_ASSERTION(mRequestedShutdownContexts.IsEmpty(),
                "shouldn't be waiting on other threads to shutdown");
 
-  MaybeRemoveFromThreadList();
+  // We shouldn't need to lock before checking isInList at this point. We're
+  // destroying the last reference to this object, so there's no way for anyone
+  // else to remove it in the middle of our check. And the not-in-list state is
+  // determined the element's next and previous members pointing to itself, so a
+  // non-atomic update to an adjacent member won't affect the outcome either.
+  if (isInList()) {
+    OffTheBooksMutexAutoLock mal(ThreadListMutex());
+    removeFrom(ThreadList());
+  }
 
 #ifdef DEBUG
   // We deliberately leak these so they can be tracked by the leak checker.
@@ -699,9 +647,6 @@ nsThread::~nsThread()
 nsresult
 nsThread::Init(const nsACString& aName)
 {
-  MOZ_ASSERT(mEvents);
-  MOZ_ASSERT(mEventTarget);
-
   // spawn thread and wait until it is fully setup
   RefPtr<nsThreadStartupEvent> startup = new nsThreadStartupEvent();
 
@@ -750,9 +695,6 @@ nsThread::InitCurrentThread()
 NS_IMETHODIMP
 nsThread::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
 {
-  MOZ_ASSERT(mEventTarget);
-  NS_ENSURE_TRUE(mEventTarget, NS_ERROR_NOT_IMPLEMENTED);
-
   nsCOMPtr<nsIRunnable> event(aEvent);
   return mEventTarget->Dispatch(event.forget(), aFlags);
 }
@@ -760,9 +702,6 @@ nsThread::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
 NS_IMETHODIMP
 nsThread::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
 {
-  MOZ_ASSERT(mEventTarget);
-  NS_ENSURE_TRUE(mEventTarget, NS_ERROR_NOT_IMPLEMENTED);
-
   LOG(("THRD(%p) Dispatch [%p %x]\n", this, /* XXX aEvent */nullptr, aFlags));
 
   return mEventTarget->Dispatch(std::move(aEvent), aFlags);
@@ -771,20 +710,13 @@ nsThread::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
 NS_IMETHODIMP
 nsThread::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aDelayMs)
 {
-  MOZ_ASSERT(mEventTarget);
-  NS_ENSURE_TRUE(mEventTarget, NS_ERROR_NOT_IMPLEMENTED);
-
   return mEventTarget->DelayedDispatch(std::move(aEvent), aDelayMs);
 }
 
 NS_IMETHODIMP
 nsThread::IsOnCurrentThread(bool* aResult)
 {
-  if (mEventTarget) {
-    return mEventTarget->IsOnCurrentThread(aResult);
-  }
-  *aResult = GetCurrentVirtualThread() == mVirtualThread;
-  return NS_OK;
+  return mEventTarget->IsOnCurrentThread(aResult);
 }
 
 NS_IMETHODIMP_(bool)
@@ -836,8 +768,6 @@ nsThread::AsyncShutdown()
 nsThreadShutdownContext*
 nsThread::ShutdownInternal(bool aSync)
 {
-  MOZ_ASSERT(mEvents);
-  MOZ_ASSERT(mEventTarget);
   MOZ_ASSERT(mThread);
   MOZ_ASSERT(mThread != PR_GetCurrentThread());
   if (NS_WARN_IF(mThread == PR_GetCurrentThread())) {
@@ -849,7 +779,12 @@ nsThread::ShutdownInternal(bool aSync)
     return nullptr;
   }
 
-  MaybeRemoveFromThreadList();
+  {
+    OffTheBooksMutexAutoLock mal(ThreadListMutex());
+    if (isInList()) {
+      removeFrom(ThreadList());
+    }
+  }
 
   NotNull<nsThread*> currentThread =
     WrapNotNull(nsThreadManager::get().GetCurrentThread());
@@ -874,12 +809,15 @@ nsThread::ShutdownInternal(bool aSync)
 void
 nsThread::ShutdownComplete(NotNull<nsThreadShutdownContext*> aContext)
 {
-  MOZ_ASSERT(mEvents);
-  MOZ_ASSERT(mEventTarget);
   MOZ_ASSERT(mThread);
   MOZ_ASSERT(aContext->mTerminatingThread == this);
 
-  MaybeRemoveFromThreadList();
+  {
+    OffTheBooksMutexAutoLock mal(ThreadListMutex());
+    if (isInList()) {
+      removeFrom(ThreadList());
+    }
+  }
 
   if (aContext->mAwaitingShutdownAck) {
     // We're in a synchronous shutdown, so tell whatever is up the stack that
@@ -1094,9 +1032,6 @@ nsThread::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
 NS_IMETHODIMP
 nsThread::ProcessNextEvent(bool aMayWait, bool* aResult)
 {
-  MOZ_ASSERT(mEvents);
-  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
-
   LOG(("THRD(%p) ProcessNextEvent [%u %u]\n", this, aMayWait,
        mNestedEventLoopDepth));
 
@@ -1329,9 +1264,6 @@ nsThread::AdjustPriority(int32_t aDelta)
 NS_IMETHODIMP
 nsThread::GetObserver(nsIThreadObserver** aObs)
 {
-  MOZ_ASSERT(mEvents);
-  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
-
   nsCOMPtr<nsIThreadObserver> obs = mEvents->GetObserver();
   obs.forget(aObs);
   return NS_OK;
@@ -1340,9 +1272,6 @@ nsThread::GetObserver(nsIThreadObserver** aObs)
 NS_IMETHODIMP
 nsThread::SetObserver(nsIThreadObserver* aObs)
 {
-  MOZ_ASSERT(mEvents);
-  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
-
   if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
     return NS_ERROR_NOT_SAME_THREAD;
   }
@@ -1361,9 +1290,6 @@ nsThread::RecursionDepth() const
 NS_IMETHODIMP
 nsThread::AddObserver(nsIThreadObserver* aObserver)
 {
-  MOZ_ASSERT(mEvents);
-  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
-
   if (NS_WARN_IF(!aObserver)) {
     return NS_ERROR_INVALID_ARG;
   }
@@ -1379,9 +1305,6 @@ nsThread::AddObserver(nsIThreadObserver* aObserver)
 NS_IMETHODIMP
 nsThread::RemoveObserver(nsIThreadObserver* aObserver)
 {
-  MOZ_ASSERT(mEvents);
-  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
-
   if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
     return NS_ERROR_NOT_SAME_THREAD;
   }
diff --git a/xpcom/threads/nsThread.h b/xpcom/threads/nsThread.h
index a447f27fef0f..4d4573bf0f92 100644
--- a/xpcom/threads/nsThread.h
+++ b/xpcom/threads/nsThread.h
@@ -60,10 +60,6 @@ public:
            MainThreadFlag aMainThread,
            uint32_t aStackSize);
 
-private:
-  nsThread();
-
-public:
   // Initialize this as a wrapper for a new PRThread, and optionally give it a name.
   nsresult Init(const nsACString& aName = NS_LITERAL_CSTRING(""));
 
@@ -150,8 +146,6 @@ public:
 
   static nsThreadEnumerator Enumerate();
 
-  static uint32_t MaxActiveThreads();
-
 private:
   void DoMainThreadSpecificProcessing(bool aReallyWait);
 
@@ -180,21 +174,6 @@ protected:
   static mozilla::LinkedList<nsThread>& ThreadList();
   static void ClearThreadList();
 
-  // The current number of active threads.
-  static uint32_t sActiveThreads;
-  // The maximum current number of active threads we've had in this session.
-  static uint32_t sMaxActiveThreads;
-
-  void AddToThreadList();
-  void MaybeRemoveFromThreadList();
-
-
-  // Whether or not these members have a value determines whether the nsThread
-  // is treated as a full XPCOM thread or as a thin wrapper.
-  //
-  // For full nsThreads, they will always contain valid pointers. For thin
-  // wrappers around non-XPCOM threads, they will be null, and event dispatch
-  // methods which rely on them will fail (and assert) if called.
   RefPtr<mozilla::SynchronizedEventQueue> mEvents;
   RefPtr<mozilla::ThreadEventTarget> mEventTarget;
 
diff --git a/xpcom/threads/nsThreadManager.cpp b/xpcom/threads/nsThreadManager.cpp
index 27a46acec6b5..7df2d9ed86f7 100644
--- a/xpcom/threads/nsThreadManager.cpp
+++ b/xpcom/threads/nsThreadManager.cpp
@@ -109,6 +109,8 @@ nsThreadManager::ReleaseThread(void* aData)
 
   auto* thread = static_cast<nsThread*>(aData);
 
+  get().UnregisterCurrentThread(*thread, true);
+
   if (thread->mHasTLSEntry) {
     thread->mHasTLSEntry = false;
     thread->Release();
@@ -319,11 +321,14 @@ nsThreadManager::Shutdown()
 
   {
     // We gather the threads from the hashtable into a list, so that we avoid
-    // holding the enumerator lock while calling nsIThread::Shutdown.
-    nsTArray<RefPtr<nsThread>> threadsToShutdown;
-    for (auto* thread : nsThread::Enumerate()) {
-      if (thread->ShutdownRequired()) {
-        threadsToShutdown.AppendElement(thread);
+    // holding the hashtable lock while calling nsIThread::Shutdown.
+    nsThreadArray threads;
+    {
+      OffTheBooksMutexAutoLock lock(mLock);
+      for (auto iter = mThreadsByPRThread.Iter(); !iter.Done(); iter.Next()) {
+        RefPtr<nsThread>& thread = iter.Data();
+        threads.AppendElement(WrapNotNull(thread));
+        iter.Remove();
       }
     }
 
@@ -337,8 +342,11 @@ nsThreadManager::Shutdown()
     // world until such time as the threads exit.
 
     // Shutdown all threads that require it (join with threads that we created).
-    for (auto& thread : threadsToShutdown) {
-      thread->Shutdown();
+    for (uint32_t i = 0; i < threads.Length(); ++i) {
+      NotNull<nsThread*> thread = threads[i];
+      if (thread->ShutdownRequired()) {
+        thread->Shutdown();
+      }
     }
   }
 
@@ -353,6 +361,12 @@ nsThreadManager::Shutdown()
 
   // There are no more background threads at this point.
 
+  // Clear the table of threads.
+  {
+    OffTheBooksMutexAutoLock lock(mLock);
+    mThreadsByPRThread.Clear();
+  }
+
   // Normally thread shutdown clears the observer for the thread, but since the
   // main thread is special we do it manually here after we're sure all events
   // have been processed.
@@ -388,15 +402,35 @@ nsThreadManager::RegisterCurrentThread(nsThread& aThread)
 {
   MOZ_ASSERT(aThread.GetPRThread() == PR_GetCurrentThread(), "bad aThread");
 
+  OffTheBooksMutexAutoLock lock(mLock);
+
+  ++mCurrentNumberOfThreads;
+  if (mCurrentNumberOfThreads > mHighestNumberOfThreads) {
+    mHighestNumberOfThreads = mCurrentNumberOfThreads;
+  }
+
+  mThreadsByPRThread.Put(aThread.GetPRThread(), &aThread);  // XXX check OOM?
+
   aThread.AddRef();  // for TLS entry
   aThread.mHasTLSEntry = true;
   PR_SetThreadPrivate(mCurThreadIndex, &aThread);
 }
 
 void
-nsThreadManager::UnregisterCurrentThread(nsThread& aThread)
+nsThreadManager::UnregisterCurrentThread(nsThread& aThread, bool aIfExists)
 {
-  MOZ_ASSERT(aThread.GetPRThread() == PR_GetCurrentThread(), "bad aThread");
+  {
+    OffTheBooksMutexAutoLock lock(mLock);
+
+    if (aIfExists && !mThreadsByPRThread.GetWeak(aThread.GetPRThread())) {
+      return;
+    }
+
+    MOZ_ASSERT(aThread.GetPRThread() == PR_GetCurrentThread(), "bad aThread");
+
+    --mCurrentNumberOfThreads;
+    mThreadsByPRThread.Remove(aThread.GetPRThread());
+  }
 
   PR_SetThreadPrivate(mCurThreadIndex, nullptr);
   // Ref-count balanced via ReleaseThread
@@ -413,6 +447,7 @@ nsThreadManager::CreateCurrentThread(SynchronizedEventQueue* aQueue,
     return nullptr;
   }
 
+  // OK, that's fine.  We'll dynamically create one :-)
   RefPtr<nsThread> thread = new nsThread(WrapNotNull(aQueue), aMainThread, 0);
   if (!thread || NS_FAILED(thread->InitCurrentThread())) {
     return nullptr;
@@ -435,11 +470,9 @@ nsThreadManager::GetCurrentThread()
   }
 
   // OK, that's fine.  We'll dynamically create one :-)
-  //
-  // We assume that if we're implicitly creating a thread here that it doesn't
-  // want an event queue. Any thread which wants an event queue should
-  // explicitly create its nsThread wrapper.
-  RefPtr<nsThread> thread = new nsThread();
+  RefPtr<ThreadEventQueue<EventQueue>> queue =
+    new ThreadEventQueue<EventQueue>(MakeUnique<EventQueue>());
+  RefPtr<nsThread> thread = new nsThread(WrapNotNull(queue), nsThread::NOT_MAIN_THREAD, 0);
   if (!thread || NS_FAILED(thread->InitCurrentThread())) {
     return nullptr;
   }
@@ -454,7 +487,7 @@ nsThreadManager::IsNSThread() const
     return false;
   }
   if (auto* thread = (nsThread*)PR_GetThreadPrivate(mCurThreadIndex)) {
-    return thread->EventQueue();
+    return thread->mShutdownRequired;
   }
   return false;
 }
@@ -503,6 +536,27 @@ nsThreadManager::NewNamedThread(const nsACString& aName,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsThreadManager::GetThreadFromPRThread(PRThread* aThread, nsIThread** aResult)
+{
+  // Keep this functioning during Shutdown
+  if (NS_WARN_IF(!mMainThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  if (NS_WARN_IF(!aThread)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  RefPtr<nsThread> temp;
+  {
+    OffTheBooksMutexAutoLock lock(mLock);
+    mThreadsByPRThread.Get(aThread, getter_AddRefs(temp));
+  }
+
+  NS_IF_ADDREF(*aResult = temp);
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsThreadManager::GetMainThread(nsIThread** aResult)
 {
@@ -605,7 +659,8 @@ nsThreadManager::GetSystemGroupEventTarget(nsIEventTarget** aTarget)
 uint32_t
 nsThreadManager::GetHighestNumberOfThreads()
 {
-  return nsThread::MaxActiveThreads();
+  OffTheBooksMutexAutoLock lock(mLock);
+  return mHighestNumberOfThreads;
 }
 
 NS_IMETHODIMP
diff --git a/xpcom/threads/nsThreadManager.h b/xpcom/threads/nsThreadManager.h
index 547c1c16a777..dd936be11081 100644
--- a/xpcom/threads/nsThreadManager.h
+++ b/xpcom/threads/nsThreadManager.h
@@ -9,6 +9,7 @@
 
 #include "mozilla/Mutex.h"
 #include "nsIThreadManager.h"
+#include "nsRefPtrHashtable.h"
 #include "nsThread.h"
 
 class nsIRunnable;
@@ -35,7 +36,7 @@ public:
 
   // Called by nsThread to inform the ThreadManager it is going away.  This
   // method must be called when the given thread is the current thread.
-  void UnregisterCurrentThread(nsThread& aThread);
+  void UnregisterCurrentThread(nsThread& aThread, bool aIfExists = false);
 
   // Returns the current thread.  Returns null if OOM or if ThreadManager isn't
   // initialized.  Creates the nsThread if one does not exist yet.
@@ -73,7 +74,10 @@ private:
   nsThreadManager()
     : mCurThreadIndex(0)
     , mMainPRThread(nullptr)
+    , mLock("nsThreadManager.mLock")
     , mInitialized(false)
+    , mCurrentNumberOfThreads(1)
+    , mHighestNumberOfThreads(1)
   {
   }
 
@@ -83,12 +87,19 @@ private:
 
   static void ReleaseThread(void* aData);
 
+  nsRefPtrHashtable<nsPtrHashKey<PRThread>, nsThread> mThreadsByPRThread;
   unsigned            mCurThreadIndex;  // thread-local-storage index
   RefPtr<nsThread>  mMainThread;
   PRThread*         mMainPRThread;
+  mozilla::OffTheBooksMutex mLock;  // protects tables
   mozilla::Atomic<bool,
                   mozilla::SequentiallyConsistent,
                   mozilla::recordreplay::Behavior::DontPreserve> mInitialized;
+
+  // The current number of threads
+  uint32_t            mCurrentNumberOfThreads;
+  // The highest number of threads encountered so far during the session
+  uint32_t            mHighestNumberOfThreads;
 };
 
 #define NS_THREADMANAGER_CID                       \