diff --git a/js/src/threading/ProtectedData.h b/js/src/threading/ProtectedData.h
index 2298dd6511f2..4f5f928de39f 100644
--- a/js/src/threading/ProtectedData.h
+++ b/js/src/threading/ProtectedData.h
@@ -7,6 +7,7 @@
 #ifndef threading_ProtectedData_h
 #define threading_ProtectedData_h
 
+#include "mozilla/Atomics.h"
 #include "jstypes.h"
 #include "threading/LockGuard.h"
 #include "threading/Mutex.h"
diff --git a/mozglue/misc/AutoProfilerLabel.cpp b/mozglue/misc/AutoProfilerLabel.cpp
index 24041ff51c0e..9bfdc19ef76e 100644
--- a/mozglue/misc/AutoProfilerLabel.cpp
+++ b/mozglue/misc/AutoProfilerLabel.cpp
@@ -6,6 +6,7 @@
 
 #include "mozilla/AutoProfilerLabel.h"
 
+#include "mozilla/Assertions.h"
 #include "mozilla/PlatformMutex.h"
 
 namespace mozilla {
diff --git a/mozglue/misc/Mutex_posix.cpp b/mozglue/misc/Mutex_posix.cpp
index 4c9e2bf5b7bd..a7a2b2512a24 100644
--- a/mozglue/misc/Mutex_posix.cpp
+++ b/mozglue/misc/Mutex_posix.cpp
@@ -5,15 +5,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/Assertions.h"
-#include "mozilla/Maybe.h"
 
-#include <algorithm>
 #include <errno.h>
 #include <pthread.h>
 #include <stdio.h>
-#include <unistd.h>
+
+#if defined(XP_DARWIN)
+#  include <pthread_spis.h>
+#endif
 
 #include "mozilla/PlatformMutex.h"
+#include "mozilla/Unused.h"
 #include "MutexPlatformData_posix.h"
 
 #define REPORT_PTHREADS_ERROR(result, msg) \
@@ -31,53 +33,22 @@
     }                                     \
   }
 
-#ifdef XP_DARWIN
-
-// CPU count. Read concurrently from multiple threads. Written once during the
-// first mutex initialization; re-initialization is safe hence relaxed ordering
-// is OK.
-static mozilla::Atomic<uint32_t, mozilla::MemoryOrdering::Relaxed> sCPUCount(0);
-
-static void EnsureCPUCount() {
-  if (sCPUCount) {
-    return;
-  }
-
-  // _SC_NPROCESSORS_CONF and _SC_NPROCESSORS_ONLN are common, but not
-  // standard.
-#  if defined(_SC_NPROCESSORS_CONF)
-  long n = sysconf(_SC_NPROCESSORS_CONF);
-  sCPUCount = (n > 0) ? uint32_t(n) : 1;
-#  elif defined(_SC_NPROCESSORS_ONLN)
-  long n = sysconf(_SC_NPROCESSORS_ONLN);
-  sCPUCount = (n > 0) ? uint32_t(n) : 1;
-#  else
-  sCPUCount = 1;
-#  endif
-}
-
-#endif  // XP_DARWIN
-
-mozilla::detail::MutexImpl::MutexImpl()
-#ifdef XP_DARWIN
-    : averageSpins(0)
-#endif
-{
+mozilla::detail::MutexImpl::MutexImpl() {
   pthread_mutexattr_t* attrp = nullptr;
 
-  // Linux with glibc and FreeBSD support adaptive mutexes that spin
-  // for a short number of tries before sleeping.  NSPR's locks did
-  // this, too, and it seems like a reasonable thing to do.
-#if (defined(__linux__) && defined(__GLIBC__)) || defined(__FreeBSD__)
-#  define ADAPTIVE_MUTEX_SUPPORTED
+#if defined(DEBUG)
+#  define MUTEX_KIND PTHREAD_MUTEX_ERRORCHECK
+// Linux with glibc, FreeBSD and macOS 10.14+ support adaptive mutexes that
+// spin for a short number of tries before sleeping.  NSPR's locks did this,
+// too, and it seems like a reasonable thing to do.
+#elif (defined(__linux__) && defined(__GLIBC__)) || defined(__FreeBSD__)
+#  define MUTEX_KIND PTHREAD_MUTEX_ADAPTIVE_NP
+#elif defined(XP_DARWIN)
+#  define POLICY_KIND _PTHREAD_MUTEX_POLICY_FIRSTFIT
 #endif
 
-#if defined(DEBUG)
+#if defined(MUTEX_KIND) || defined(POLICY_KIND)
 #  define ATTR_REQUIRED
-#  define MUTEX_KIND PTHREAD_MUTEX_ERRORCHECK
-#elif defined(ADAPTIVE_MUTEX_SUPPORTED)
-#  define ATTR_REQUIRED
-#  define MUTEX_KIND PTHREAD_MUTEX_ADAPTIVE_NP
 #endif
 
 #if defined(ATTR_REQUIRED)
@@ -87,9 +58,14 @@ mozilla::detail::MutexImpl::MutexImpl()
       pthread_mutexattr_init(&attr),
       "mozilla::detail::MutexImpl::MutexImpl: pthread_mutexattr_init failed");
 
+#  if defined(MUTEX_KIND)
   TRY_CALL_PTHREADS(pthread_mutexattr_settype(&attr, MUTEX_KIND),
                     "mozilla::detail::MutexImpl::MutexImpl: "
                     "pthread_mutexattr_settype failed");
+#  elif defined(POLICY_KIND)
+  // This can fail on macOS 10.13 and lower but that's OK
+  Unused << pthread_mutexattr_setpolicy_np(&attr, POLICY_KIND);
+#  endif
   attrp = &attr;
 #endif
 
@@ -102,10 +78,6 @@ mozilla::detail::MutexImpl::MutexImpl()
                     "mozilla::detail::MutexImpl::MutexImpl: "
                     "pthread_mutexattr_destroy failed");
 #endif
-
-#ifdef XP_DARWIN
-  EnsureCPUCount();
-#endif
 }
 
 mozilla::detail::MutexImpl::~MutexImpl() {
@@ -137,49 +109,7 @@ bool mozilla::detail::MutexImpl::mutexTryLock() {
       "mozilla::detail::MutexImpl::mutexTryLock: pthread_mutex_trylock failed");
 }
 
-void mozilla::detail::MutexImpl::lock() {
-#ifndef XP_DARWIN
-  mutexLock();
-#else
-  // Mutex performance on OSX can be very poor if there's a lot of contention as
-  // this causes excessive context switching. On Linux/FreeBSD we use the
-  // adaptive mutex type (PTHREAD_MUTEX_ADAPTIVE_NP) to address this, but this
-  // isn't available on OSX. The code below is a reimplementation of this
-  // feature.
-
-  MOZ_ASSERT(sCPUCount);
-  if (sCPUCount == 1) {
-    mutexLock();
-    return;
-  }
-
-  if (!mutexTryLock()) {
-    const int32_t SpinLimit = 100;
-
-    int32_t count = 0;
-    int32_t maxSpins = std::min(SpinLimit, 2 * averageSpins + 10);
-    do {
-      if (count >= maxSpins) {
-        mutexLock();
-        break;
-      }
-      // Hint to the processor that we're spinning.
-#  ifdef __x86_64__
-#    define SPIN_HINT "pause"
-#  elif defined(__aarch64__)
-#    define SPIN_HINT "yield"
-#  endif
-      asm volatile(SPIN_HINT ::: "memory");
-#  undef SPIN_HINT
-      count++;
-    } while (!mutexTryLock());
-
-    // Update moving average.
-    averageSpins += (count - averageSpins) / 8;
-    MOZ_ASSERT(averageSpins >= 0 && averageSpins <= SpinLimit);
-  }
-#endif  // XP_DARWIN
-}
+void mozilla::detail::MutexImpl::lock() { mutexLock(); }
 
 void mozilla::detail::MutexImpl::unlock() {
   TRY_CALL_PTHREADS(
diff --git a/mozglue/misc/PlatformMutex.h b/mozglue/misc/PlatformMutex.h
index aaa73ec6dc2b..704bf60e412a 100644
--- a/mozglue/misc/PlatformMutex.h
+++ b/mozglue/misc/PlatformMutex.h
@@ -9,7 +9,6 @@
 
 #include <utility>
 
-#include "mozilla/Atomics.h"
 #include "mozilla/Attributes.h"
 
 #if !defined(XP_WIN)
@@ -53,12 +52,6 @@ class MutexImpl {
   static_assert(sizeof(pthread_mutex_t) / sizeof(void*) != 0 &&
                     sizeof(pthread_mutex_t) % sizeof(void*) == 0,
                 "pthread_mutex_t must have pointer alignment");
-#  ifdef XP_DARWIN
-  // Moving average of the number of spins it takes to acquire the mutex if we
-  // have to wait. May be accessed by multiple threads concurrently. Getting the
-  // latest value is not essential hence relaxed memory ordering is sufficient.
-  mozilla::Atomic<int32_t, mozilla::MemoryOrdering::Relaxed> averageSpins;
-#  endif
 #else
   void* platformData_[6];
 #endif