diff --git a/dom/base/test/test_frameLoader_switchProcess.html b/dom/base/test/test_frameLoader_switchProcess.html
index 1b96d323d7f0..10c27efee57f 100644
--- a/dom/base/test/test_frameLoader_switchProcess.html
+++ b/dom/base/test/test_frameLoader_switchProcess.html
@@ -68,10 +68,7 @@
   }
 
   SpecialPowers.pushPrefEnv(
-    // XXX Set LRUPoolLevels to 2 to avoid breaking priority tests
-    { "set": [["dom.ipc.processPriorityManager.BACKGROUND.LRUPoolLevels", 2],
-              ["dom.ipc.processPriorityManager.BACKGROUND_PERCEIVABLE.LRUPoolLevels", 2],
-              ["dom.ipc.processPriorityManager.testMode", true],
+    { "set": [["dom.ipc.processPriorityManager.testMode", true],
               ["dom.ipc.processPriorityManager.enabled", true],
               ["dom.ipc.tabs.disabled", false],
               ["dom.ipc.processCount", 3],
diff --git a/dom/ipc/ProcessPriorityManager.cpp b/dom/ipc/ProcessPriorityManager.cpp
index 76889f5418ff..e937bda6d2bb 100644
--- a/dom/ipc/ProcessPriorityManager.cpp
+++ b/dom/ipc/ProcessPriorityManager.cpp
@@ -144,6 +144,7 @@ private:
 class ProcessPriorityManagerImpl final
   : public nsIObserver
   , public WakeLockObserver
+  , public nsSupportsWeakReference
 {
 public:
   /**
@@ -154,6 +155,7 @@ public:
 
   static void StaticInit();
   static bool PrefsEnabled();
+  static bool TestMode();
 
   NS_DECL_ISUPPORTS
   NS_DECL_NSIOBSERVER
@@ -202,7 +204,16 @@ public:
    */
   void Unfreeze();
 
+  /**
+   * Call ShutDown before destroying the ProcessPriorityManager because
+   * WakeLockObserver hols a strong reference to it.
+   */
+  void ShutDown();
+
 private:
+  static bool sPrefsEnabled;
+  static bool sRemoteTabsDisabled;
+  static bool sTestMode;
   static bool sPrefListenersRegistered;
   static bool sInitialized;
   static bool sFrozen;
@@ -288,6 +299,7 @@ public:
   NS_DECL_NSITIMERCALLBACK
 
   virtual void Notify(const WakeLockInformation& aInfo) override;
+  static void StaticInit();
   void Init();
 
   int32_t Pid() const;
@@ -318,7 +330,12 @@ public:
   ProcessPriority CurrentPriority();
   ProcessPriority ComputePriority();
 
-  void ScheduleResetPriority(const char* aTimeoutPref);
+  enum TimeoutPref {
+    BACKGROUND_PERCEIVABLE_GRACE_PERIOD,
+    BACKGROUND_GRACE_PERIOD,
+  };
+
+  void ScheduleResetPriority(TimeoutPref aTimeoutPref);
   void ResetPriority();
   void ResetPriorityNow();
   void SetPriorityNow(ProcessPriority aPriority, uint32_t aLRU = 0);
@@ -328,6 +345,9 @@ public:
   void ShutDown();
 
 private:
+  static uint32_t sBackgroundPerceivableGracePeriodMS;
+  static uint32_t sBackgroundGracePeriodMS;
+
   void FireTestOnlyObserverNotification(
     const char* aTopic,
     const nsACString& aData = EmptyCString());
@@ -354,26 +374,42 @@ private:
 };
 
 /* static */ bool ProcessPriorityManagerImpl::sInitialized = false;
+/* static */ bool ProcessPriorityManagerImpl::sPrefsEnabled = false;
+/* static */ bool ProcessPriorityManagerImpl::sRemoteTabsDisabled = true;
+/* static */ bool ProcessPriorityManagerImpl::sTestMode = false;
 /* static */ bool ProcessPriorityManagerImpl::sPrefListenersRegistered = false;
 /* static */ bool ProcessPriorityManagerImpl::sFrozen = false;
 /* static */ StaticRefPtr<ProcessPriorityManagerImpl>
   ProcessPriorityManagerImpl::sSingleton;
+/* static */ uint32_t ParticularProcessPriorityManager::sBackgroundPerceivableGracePeriodMS = 0;
+/* static */ uint32_t ParticularProcessPriorityManager::sBackgroundGracePeriodMS = 0;
 
 NS_IMPL_ISUPPORTS(ProcessPriorityManagerImpl,
-                  nsIObserver);
+                  nsIObserver,
+                  nsISupportsWeakReference);
 
 /* static */ void
 ProcessPriorityManagerImpl::PrefChangedCallback(const char* aPref,
                                                 void* aClosure)
 {
   StaticInit();
+  if (!PrefsEnabled() && sSingleton) {
+    sSingleton->ShutDown();
+    sSingleton = nullptr;
+    sInitialized = false;
+  }
 }
 
 /* static */ bool
 ProcessPriorityManagerImpl::PrefsEnabled()
 {
-  return Preferences::GetBool("dom.ipc.processPriorityManager.enabled") &&
-         !Preferences::GetBool("dom.ipc.tabs.disabled");
+  return sPrefsEnabled && !sRemoteTabsDisabled;
+}
+
+/* static */ bool
+ProcessPriorityManagerImpl::TestMode()
+{
+  return sTestMode;
 }
 
 /* static */ void
@@ -389,6 +425,15 @@ ProcessPriorityManagerImpl::StaticInit()
     return;
   }
 
+  if (!sPrefListenersRegistered) {
+    Preferences::AddBoolVarCache(&sPrefsEnabled,
+                                 "dom.ipc.processPriorityManager.enabled");
+    Preferences::AddBoolVarCache(&sRemoteTabsDisabled,
+                                 "dom.ipc.tabs.disabled");
+    Preferences::AddBoolVarCache(&sTestMode,
+                                 "dom.ipc.processPriorityManager.testMode");
+  }
+
   // If IPC tabs aren't enabled at startup, don't bother with any of this.
   if (!PrefsEnabled()) {
     LOG("InitProcessPriorityManager bailing due to prefs.");
@@ -432,6 +477,12 @@ ProcessPriorityManagerImpl::ProcessPriorityManagerImpl()
 }
 
 ProcessPriorityManagerImpl::~ProcessPriorityManagerImpl()
+{
+  ShutDown();
+}
+
+void
+ProcessPriorityManagerImpl::ShutDown()
 {
   UnregisterWakeLockObserver(this);
 }
@@ -448,9 +499,9 @@ ProcessPriorityManagerImpl::Init()
 
   nsCOMPtr<nsIObserverService> os = services::GetObserverService();
   if (os) {
-    os->AddObserver(this, "ipc:content-created", /* ownsWeak */ false);
-    os->AddObserver(this, "ipc:content-shutdown", /* ownsWeak */ false);
-    os->AddObserver(this, "screen-state-changed", /* ownsWeak */ false);
+    os->AddObserver(this, "ipc:content-created", /* ownsWeak */ true);
+    os->AddObserver(this, "ipc:content-shutdown", /* ownsWeak */ true);
+    os->AddObserver(this, "screen-state-changed", /* ownsWeak */ true);
   }
 }
 
@@ -664,6 +715,15 @@ ParticularProcessPriorityManager::ParticularProcessPriorityManager(
   LOGP("Creating ParticularProcessPriorityManager.");
 }
 
+void
+ParticularProcessPriorityManager::StaticInit()
+{
+  Preferences::AddUintVarCache(&sBackgroundPerceivableGracePeriodMS,
+                               "dom.ipc.processPriorityManager.backgroundPerceivableGracePeriodMS");
+  Preferences::AddUintVarCache(&sBackgroundGracePeriodMS,
+                               "dom.ipc.processPriorityManager.backgroundGracePeriodMS");
+}
+
 void
 ParticularProcessPriorityManager::Init()
 {
@@ -930,9 +990,9 @@ ParticularProcessPriorityManager::ResetPriority()
     // can get their next track started, if there is one, before getting
     // downgraded.
     if (mPriority == PROCESS_PRIORITY_BACKGROUND_PERCEIVABLE) {
-      ScheduleResetPriority("backgroundPerceivableGracePeriodMS");
+      ScheduleResetPriority(BACKGROUND_PERCEIVABLE_GRACE_PERIOD);
     } else {
-      ScheduleResetPriority("backgroundGracePeriodMS");
+      ScheduleResetPriority(BACKGROUND_GRACE_PERIOD);
     }
     return;
   }
@@ -947,15 +1007,26 @@ ParticularProcessPriorityManager::ResetPriorityNow()
 }
 
 void
-ParticularProcessPriorityManager::ScheduleResetPriority(const char* aTimeoutPref)
+ParticularProcessPriorityManager::ScheduleResetPriority(TimeoutPref aTimeoutPref)
 {
   if (mResetPriorityTimer) {
     LOGP("ScheduleResetPriority bailing; the timer is already running.");
     return;
   }
 
-  uint32_t timeout = Preferences::GetUint(
-    nsPrintfCString("dom.ipc.processPriorityManager.%s", aTimeoutPref).get());
+  uint32_t timeout = 0;
+  switch (aTimeoutPref) {
+    case BACKGROUND_PERCEIVABLE_GRACE_PERIOD:
+      timeout = sBackgroundPerceivableGracePeriodMS;
+      break;
+    case BACKGROUND_GRACE_PERIOD:
+      timeout = sBackgroundGracePeriodMS;
+      break;
+    default:
+      MOZ_ASSERT(false, "Unrecognized timeout pref");
+      break;
+  }
+
   LOGP("Scheduling reset timer to fire in %dms.", timeout);
   mResetPriorityTimer = do_CreateInstance("@mozilla.org/timer;1");
   mResetPriorityTimer->InitWithCallback(this, timeout, nsITimer::TYPE_ONE_SHOT);
@@ -1134,7 +1205,7 @@ ProcessPriorityManagerImpl::FireTestOnlyObserverNotification(
   const char* aTopic,
   const nsACString& aData /* = EmptyCString() */)
 {
-  if (!Preferences::GetBool("dom.ipc.processPriorityManager.testMode")) {
+  if (!TestMode()) {
     return;
   }
 
@@ -1153,7 +1224,7 @@ ParticularProcessPriorityManager::FireTestOnlyObserverNotification(
   const char* aTopic,
   const char* aData /* = nullptr */ )
 {
-  if (!Preferences::GetBool("dom.ipc.processPriorityManager.testMode")) {
+  if (!ProcessPriorityManagerImpl::TestMode()) {
     return;
   }
 
@@ -1170,7 +1241,7 @@ ParticularProcessPriorityManager::FireTestOnlyObserverNotification(
   const char* aTopic,
   const nsACString& aData /* = EmptyCString() */)
 {
-  if (!Preferences::GetBool("dom.ipc.processPriorityManager.testMode")) {
+  if (!ProcessPriorityManagerImpl::TestMode()) {
     return;
   }
 
@@ -1381,6 +1452,7 @@ ProcessPriorityManager::Init()
 {
   ProcessPriorityManagerImpl::StaticInit();
   ProcessPriorityManagerChild::StaticInit();
+  ParticularProcessPriorityManager::StaticInit();
 }
 
 /* static */ void