diff --git a/mozglue/misc/AutoProfilerLabel.cpp b/mozglue/misc/AutoProfilerLabel.cpp
index 9bfdc19ef76e..24041ff51c0e 100644
--- a/mozglue/misc/AutoProfilerLabel.cpp
+++ b/mozglue/misc/AutoProfilerLabel.cpp
@@ -6,7 +6,6 @@
 
 #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 a7a2b2512a24..4c9e2bf5b7bd 100644
--- a/mozglue/misc/Mutex_posix.cpp
+++ b/mozglue/misc/Mutex_posix.cpp
@@ -5,17 +5,15 @@
  * 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>
-
-#if defined(XP_DARWIN)
-#  include <pthread_spis.h>
-#endif
+#include <unistd.h>
 
 #include "mozilla/PlatformMutex.h"
-#include "mozilla/Unused.h"
 #include "MutexPlatformData_posix.h"
 
 #define REPORT_PTHREADS_ERROR(result, msg) \
@@ -33,22 +31,53 @@
     }                                     \
   }
 
-mozilla::detail::MutexImpl::MutexImpl() {
+#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
+{
   pthread_mutexattr_t* attrp = nullptr;
 
-#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
+  // 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
 #endif
 
-#if defined(MUTEX_KIND) || defined(POLICY_KIND)
+#if defined(DEBUG)
 #  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)
@@ -58,14 +87,9 @@ 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
 
@@ -78,6 +102,10 @@ mozilla::detail::MutexImpl::MutexImpl() {
                     "mozilla::detail::MutexImpl::MutexImpl: "
                     "pthread_mutexattr_destroy failed");
 #endif
+
+#ifdef XP_DARWIN
+  EnsureCPUCount();
+#endif
 }
 
 mozilla::detail::MutexImpl::~MutexImpl() {
@@ -109,7 +137,49 @@ bool mozilla::detail::MutexImpl::mutexTryLock() {
       "mozilla::detail::MutexImpl::mutexTryLock: pthread_mutex_trylock failed");
 }
 
-void mozilla::detail::MutexImpl::lock() { mutexLock(); }
+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::unlock() {
   TRY_CALL_PTHREADS(
diff --git a/mozglue/misc/PlatformMutex.h b/mozglue/misc/PlatformMutex.h
index 704bf60e412a..aaa73ec6dc2b 100644
--- a/mozglue/misc/PlatformMutex.h
+++ b/mozglue/misc/PlatformMutex.h
@@ -9,6 +9,7 @@
 
 #include <utility>
 
+#include "mozilla/Atomics.h"
 #include "mozilla/Attributes.h"
 
 #if !defined(XP_WIN)
@@ -52,6 +53,12 @@ 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