diff --git a/xpcom/threads/CPUUsageWatcher.cpp b/xpcom/threads/CPUUsageWatcher.cpp
index ee9731b0bf54..e00a4761d289 100644
--- a/xpcom/threads/CPUUsageWatcher.cpp
+++ b/xpcom/threads/CPUUsageWatcher.cpp
@@ -9,18 +9,15 @@
 #include "prsystem.h"
 
 #ifdef XP_MACOSX
+#include <sys/resource.h>
+#include <mach/clock.h>
 #include <mach/mach_host.h>
 #endif
 
-// We only support OSX and Windows, because on Linux we're forced to read
-// from /proc/stat in order to get global CPU values. We would prefer to not
-// eat that cost for this.
-#if defined(NIGHTLY_BUILD) && (defined(XP_WIN) || defined(XP_MACOSX))
-#define CPU_USAGE_WATCHER_ACTIVE
-#endif
-
 namespace mozilla {
 
+#ifdef CPU_USAGE_WATCHER_ACTIVE
+
 // Even if the machine only has one processor, tolerate up to 50%
 // external CPU usage.
 static const float kTolerableExternalCPUUsageFloor = 0.5f;
@@ -37,26 +34,44 @@ struct CPUStats {
 
 #ifdef XP_MACOSX
 
-static const uint64_t kNanosecondsPerSecond = 1000000000LL;
-static const uint64_t kCPUCheckInterval = kNanosecondsPerSecond / 2LL;
+static const uint64_t kMicrosecondsPerSecond = 1000000LL;
+static const uint64_t kNanosecondsPerMicrosecond = 1000LL;
+static const uint64_t kCPUCheckInterval = kMicrosecondsPerSecond / 2LL;
 
-Result<uint64_t, CPUUsageWatcherError>
-GetClockTime(clockid_t clockId) {
-  timespec clockResult;
-  bool success = !clock_gettime(clockId, &clockResult);
-  if (!success) {
-    return Err(ClockGetTimeError);
-  }
-  return ((uint64_t)clockResult.tv_sec) * kNanosecondsPerSecond +
-         (uint64_t)clockResult.tv_nsec;
+uint64_t GetMicroseconds(timeval time) {
+    return ((uint64_t)time.tv_sec) * kMicrosecondsPerSecond +
+           (uint64_t)time.tv_usec;
+}
+
+uint64_t GetMicroseconds(mach_timespec_t time) {
+    return ((uint64_t)time.tv_sec) * kMicrosecondsPerSecond +
+           ((uint64_t)time.tv_nsec) / kNanosecondsPerMicrosecond;
 }
 
 Result<CPUStats, CPUUsageWatcherError>
 GetProcessCPUStats(int32_t numCPUs) {
   CPUStats result = {};
-  MOZ_TRY_VAR(result.usageTime, GetClockTime(CLOCK_PROCESS_CPUTIME_ID));
-  MOZ_TRY_VAR(result.updateTime, GetClockTime(CLOCK_MONOTONIC));
-  // CLOCK_PROCESS_CPUTIME_ID will give us the sum of the values across all
+  rusage usage;
+  int32_t rusageResult = getrusage(RUSAGE_SELF, &usage);
+  if (rusageResult == -1) {
+    return Err(GetProcessTimesError);
+  }
+  result.usageTime = GetMicroseconds(usage.ru_utime) + GetMicroseconds(usage.ru_stime);
+
+  clock_serv_t realtimeClock;
+  kern_return_t errorResult =
+      host_get_clock_service(mach_host_self(), REALTIME_CLOCK, &realtimeClock);
+  if (errorResult != KERN_SUCCESS) {
+    return Err(GetProcessTimesError);
+  }
+  mach_timespec_t time;
+  errorResult = clock_get_time(realtimeClock, &time);
+  if (errorResult != KERN_SUCCESS) {
+    return Err(GetProcessTimesError);
+  }
+  result.updateTime = GetMicroseconds(time);
+
+  // getrusage will give us the sum of the values across all
   // of our cores. Divide by the number of CPUs to get an average.
   result.usageTime /= numCPUs;
   return result;
@@ -157,8 +172,6 @@ CPUUsageWatcher::Init()
   mExternalUsageThreshold = std::max(1.0f - 1.0f / (float)mNumCPUs,
                                      kTolerableExternalCPUUsageFloor);
 
-#ifdef CPU_USAGE_WATCHER_ACTIVE
-
   CPUStats processTimes;
   MOZ_TRY_VAR(processTimes, GetProcessCPUStats(mNumCPUs));
   mProcessUpdateTime = processTimes.updateTime;
@@ -176,18 +189,16 @@ CPUUsageWatcher::Init()
     NS_NewRunnableFunction("CPUUsageWatcher::Init",
                            [=]() { HangMonitor::RegisterAnnotator(*self); }));
 
-#endif // CPU_USAGE_WATCHER_ACTIVE
   return Ok();
 }
 
 void
 CPUUsageWatcher::Uninit()
 {
+  if (mInitialized) {
+    HangMonitor::UnregisterAnnotator(*this);
+  }
   mInitialized = false;
-
-#ifdef CPU_USAGE_WATCHER_ACTIVE
-  HangMonitor::UnregisterAnnotator(*this);
-#endif // CPU_USAGE_WATCHER_ACTIVE
 }
 
 Result<Ok, CPUUsageWatcherError>
@@ -197,7 +208,6 @@ CPUUsageWatcher::CollectCPUUsage()
     return Ok();
   }
 
-#ifdef CPU_USAGE_WATCHER_ACTIVE
   mExternalUsageRatio = 0.0f;
 
   CPUStats processTimes;
@@ -224,7 +234,6 @@ CPUUsageWatcher::CollectCPUUsage()
 
   mExternalUsageRatio = std::max(0.0f,
                                  globalUsageNormalized - processUsageNormalized);
-#endif // CPU_USAGE_WATCHER_ACTIVE
 
   return Ok();
 }
@@ -240,4 +249,24 @@ CPUUsageWatcher::AnnotateHang(HangMonitor::HangAnnotations& aAnnotations) {
   }
 }
 
+#else // !CPU_USAGE_WATCHER_ACTIVE
+
+Result<Ok, CPUUsageWatcherError>
+CPUUsageWatcher::Init()
+{
+  return Ok();
+}
+
+void CPUUsageWatcher::Uninit() {}
+
+Result<Ok, CPUUsageWatcherError>
+CPUUsageWatcher::CollectCPUUsage()
+{
+  return Ok();
+}
+
+void CPUUsageWatcher::AnnotateHang(HangMonitor::HangAnnotations& aAnnotations) {}
+
+#endif // CPU_USAGE_WATCHER_ACTIVE
+
 } // namespace mozilla
diff --git a/xpcom/threads/CPUUsageWatcher.h b/xpcom/threads/CPUUsageWatcher.h
index 343bf8547364..5e8e58981237 100644
--- a/xpcom/threads/CPUUsageWatcher.h
+++ b/xpcom/threads/CPUUsageWatcher.h
@@ -11,6 +11,13 @@
 
 #include "mozilla/HangAnnotations.h"
 
+// We only support OSX and Windows, because on Linux we're forced to read
+// from /proc/stat in order to get global CPU values. We would prefer to not
+// eat that cost for this.
+#if defined(NIGHTLY_BUILD) && (defined(XP_WIN) || defined(XP_MACOSX))
+#define CPU_USAGE_WATCHER_ACTIVE
+#endif
+
 namespace mozilla {
 
 enum CPUUsageWatcherError : uint8_t
@@ -33,6 +40,7 @@ class CPUUsageWatcher
   : public HangMonitor::Annotator
 {
 public:
+#ifdef CPU_USAGE_WATCHER_ACTIVE
   CPUUsageWatcher()
     : mInitialized(false)
     , mExternalUsageThreshold(0)
@@ -42,6 +50,7 @@ public:
     , mGlobalUsageTime(0)
     , mGlobalUpdateTime(0)
   {}
+#endif
 
   Result<Ok, CPUUsageWatcherError> Init();
 
@@ -54,6 +63,7 @@ public:
 
   void AnnotateHang(HangMonitor::HangAnnotations& aAnnotations) final;
 private:
+#ifdef CPU_USAGE_WATCHER_ACTIVE
   bool mInitialized;
   // The threshold above which we will mark a hang as occurring under high
   // external CPU usage conditions
@@ -75,6 +85,7 @@ private:
   uint64_t mGlobalUpdateTime;
   // The number of virtual cores on our machine
   uint64_t mNumCPUs;
+#endif
 };
 
 } // namespace mozilla