diff --git a/memory/build/Mutex.h b/memory/build/Mutex.h
index 3f60f1045ce7..f360084f64d0 100644
--- a/memory/build/Mutex.h
+++ b/memory/build/Mutex.h
@@ -153,13 +153,9 @@ typedef Mutex StaticMutex;
 #ifdef XP_WIN
 typedef DWORD ThreadId;
 inline ThreadId GetThreadId() { return GetCurrentThreadId(); }
-inline bool ThreadIdEqual(ThreadId a, ThreadId b) { return a == b; }
 #else
 typedef pthread_t ThreadId;
 inline ThreadId GetThreadId() { return pthread_self(); }
-inline bool ThreadIdEqual(ThreadId a, ThreadId b) {
-  return pthread_equal(a, b);
-}
 #endif
 
 class MOZ_CAPABILITY("mutex") MaybeMutex : public Mutex {
@@ -216,7 +212,7 @@ class MOZ_CAPABILITY("mutex") MaybeMutex : public Mutex {
   // protected resource.
 #ifdef MOZ_DEBUG
   bool SafeOnThisThread() const {
-    return mDoLock == MUST_LOCK || ThreadIdEqual(GetThreadId(), mThreadId);
+    return mDoLock == MUST_LOCK || GetThreadId() == mThreadId;
   }
 #endif
 
@@ -232,7 +228,7 @@ class MOZ_CAPABILITY("mutex") MaybeMutex : public Mutex {
       return true;
     }
 
-    MOZ_ASSERT(ThreadIdEqual(GetThreadId(), mThreadId));
+    MOZ_ASSERT(GetThreadId() == mThreadId);
     return false;
   }
 
diff --git a/memory/build/mozjemalloc.cpp b/memory/build/mozjemalloc.cpp
index d563074381ac..bf2ca8078668 100644
--- a/memory/build/mozjemalloc.cpp
+++ b/memory/build/mozjemalloc.cpp
@@ -1392,8 +1392,7 @@ class ArenaCollection {
 
   // We're running on the main thread which is set by a call to SetMainThread().
   bool IsOnMainThread() const {
-    return mMainThreadId.isSome() &&
-           ThreadIdEqual(mMainThreadId.value(), GetThreadId());
+    return mMainThreadId.isSome() && mMainThreadId.value() == GetThreadId();
   }
 
   // We're running on the main thread or SetMainThread() has never been called.
@@ -1402,10 +1401,11 @@ class ArenaCollection {
   }
 
   // After a fork set the new thread ID in the child.
-  void ResetMainThread() {
-    // The post fork handler in the child can run from a MacOS worker thread,
-    // so we can't set our main thread to it here.  Instead we have to clear it.
-    mMainThreadId = Nothing();
+  void PostForkFixMainThread() {
+    if (mMainThreadId.isSome()) {
+      // Only if the main thread has been defined.
+      mMainThreadId = Some(GetThreadId());
+    }
   }
 
   void SetMainThread() {
@@ -1550,9 +1550,6 @@ static bool malloc_init_hard();
 FORK_HOOK void _malloc_prefork(void);
 FORK_HOOK void _malloc_postfork_parent(void);
 FORK_HOOK void _malloc_postfork_child(void);
-#  ifdef XP_DARWIN
-FORK_HOOK void _malloc_postfork(void);
-#  endif
 #endif
 
 // End forward declarations.
@@ -5181,23 +5178,13 @@ inline void MozJemalloc::moz_set_max_dirty_page_modifier(int32_t aModifier) {
 // state for the child is if fork is called from the main thread only.  Or the
 // child must not use them, eg it should call exec().  We attempt to prevent the
 // child for accessing these arenas by refusing to re-initialise them.
-//
-// This is only accessed in the fork handlers while gArenas.mLock is held.
 static pthread_t gForkingThread;
 
-#  ifdef XP_DARWIN
-// This is only accessed in the fork handlers while gArenas.mLock is held.
-static mach_port_t gForkingProcess;
-#  endif
-
 FORK_HOOK
 void _malloc_prefork(void) MOZ_NO_THREAD_SAFETY_ANALYSIS {
   // Acquire all mutexes in a safe order.
   gArenas.mLock.Lock();
   gForkingThread = pthread_self();
-#  ifdef XP_DARWIN
-  gForkingProcess = mach_task_self();
-#  endif
 
   for (auto arena : gArenas.iter()) {
     if (arena->mLock.LockIsEnabled()) {
@@ -5228,9 +5215,6 @@ void _malloc_postfork_parent(void) MOZ_NO_THREAD_SAFETY_ANALYSIS {
 
 FORK_HOOK
 void _malloc_postfork_child(void) {
-  // Do this before iterating over the arenas.
-  gArenas.ResetMainThread();
-
   // Reinitialize all mutexes, now that fork() has completed.
   huge_mtx.Init();
 
@@ -5240,24 +5224,10 @@ void _malloc_postfork_child(void) {
     arena->mLock.Reinit(gForkingThread);
   }
 
+  gArenas.PostForkFixMainThread();
   gArenas.mLock.Init();
 }
-
-#  ifdef XP_DARWIN
-FORK_HOOK
-void _malloc_postfork(void) {
-  // On MacOS we need to check if this is running in the parent or child
-  // process.
-  bool is_in_parent = mach_task_self() == gForkingProcess;
-  gForkingProcess = 0;
-  if (is_in_parent) {
-    _malloc_postfork_parent();
-  } else {
-    _malloc_postfork_child();
-  }
-}
-#  endif  // XP_DARWIN
-#endif    // ! XP_WIN
+#endif  // XP_WIN
 
 // End library-private functions.
 // ***************************************************************************
diff --git a/memory/build/zone.c b/memory/build/zone.c
index eaabcf01bdbf..7311ccf27bc4 100644
--- a/memory/build/zone.c
+++ b/memory/build/zone.c
@@ -232,9 +232,8 @@ static void zone_print(malloc_zone_t* zone, boolean_t verbose) {}
 
 static void zone_log(malloc_zone_t* zone, void* address) {}
 
-// On Darwin the postfork handler is called in both the parent and the child.
 extern void _malloc_prefork(void);
-extern void _malloc_postfork(void);
+extern void _malloc_postfork_child(void);
 
 static void zone_force_lock(malloc_zone_t* zone) {
   // /!\ This calls into mozjemalloc. It works because we're linked in the
@@ -245,7 +244,7 @@ static void zone_force_lock(malloc_zone_t* zone) {
 static void zone_force_unlock(malloc_zone_t* zone) {
   // /!\ This calls into mozjemalloc. It works because we're linked in the
   // same library.
-  _malloc_postfork();
+  _malloc_postfork_child();
 }
 
 static void zone_statistics(malloc_zone_t* zone, malloc_statistics_t* stats) {