--- a/dom/ipc/ContentProcess.cpp
+++ b/dom/ipc/ContentProcess.cpp
@@ -122,17 +122,17 @@ ContentProcess::Init(int aArgc, char* aA
   bool isForBrowser;
 
 #if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
   // If passed in grab the profile path for sandboxing
   bool foundProfile = false;
   nsCOMPtr<nsIFile> profileDir;
 #endif
 
-  char* schedulerPrefs;
+  char* schedulerPrefs = nullptr;
   InfallibleTArray<PrefSetting> prefsArray;
   for (int idx = aArgc; idx > 0; idx--) {
     if (!aArgv[idx]) {
       continue;
     }
 
     if (!strcmp(aArgv[idx], "-appdir")) {
       MOZ_ASSERT(!foundAppdir);

--- a/xpcom/threads/Scheduler.cpp
+++ b/xpcom/threads/Scheduler.cpp
@@ -202,21 +202,21 @@ private:
   ThreadController mController;
 
   static size_t sNumThreadsRunning;
   static bool sUnlabeledEventRunning;
 
   JSContext* mContexts[CooperativeThreadPool::kMaxThreads];
 };
 
-bool SchedulerImpl::sPrefScheduler;
-bool SchedulerImpl::sPrefChaoticScheduling;
-bool SchedulerImpl::sPrefPreemption;
-bool SchedulerImpl::sPrefUseMultipleQueues;
-size_t SchedulerImpl::sPrefThreadCount;
+bool SchedulerImpl::sPrefScheduler = false;
+bool SchedulerImpl::sPrefChaoticScheduling = false;
+bool SchedulerImpl::sPrefPreemption = false;
+bool SchedulerImpl::sPrefUseMultipleQueues = false;
+size_t SchedulerImpl::sPrefThreadCount = 2;
 
 size_t SchedulerImpl::sNumThreadsRunning;
 bool SchedulerImpl::sUnlabeledEventRunning;
 
 bool
 SchedulerEventQueue::PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
                               EventPriority aPriority)
 {
@@ -756,29 +756,45 @@ Scheduler::Shutdown()
   }
 }
 
 /* static */ nsCString
 Scheduler::GetPrefs()
 {
   MOZ_ASSERT(XRE_IsParentProcess());
   nsPrintfCString result("%d%d%d%d,%d",
-                         Preferences::GetBool("dom.ipc.scheduler", false),
-                         Preferences::GetBool("dom.ipc.scheduler.chaoticScheduling", false),
-                         Preferences::GetBool("dom.ipc.scheduler.preemption", false) ,
-                         Preferences::GetBool("dom.ipc.scheduler.useMultipleQueues", false),
-                         Preferences::GetInt("dom.ipc.scheduler.threadCount", 2));
+                         Preferences::GetBool("dom.ipc.scheduler",
+                                              SchedulerImpl::sPrefScheduler),
+                         Preferences::GetBool("dom.ipc.scheduler.chaoticScheduling",
+                                              SchedulerImpl::sPrefChaoticScheduling),
+                         Preferences::GetBool("dom.ipc.scheduler.preemption",
+                                              SchedulerImpl::sPrefPreemption),
+                         Preferences::GetBool("dom.ipc.scheduler.useMultipleQueues",
+                                              SchedulerImpl::sPrefUseMultipleQueues),
+                         Preferences::GetInt("dom.ipc.scheduler.threadCount",
+                                             SchedulerImpl::sPrefThreadCount));
+
   return result;
 }
 
 /* static */ void
 Scheduler::SetPrefs(const char* aPrefs)
 {
   MOZ_ASSERT(XRE_IsContentProcess());
 
+  // If the prefs weren't sent to this process, use the default values.
+  if (!aPrefs) {
+    return;
+  }
+
+  // If the pref string appears truncated, use the default values.
+  if (strlen(aPrefs) < 6) {
+    return;
+  }
+
   SchedulerImpl::sPrefScheduler = aPrefs[0] == '1';
   SchedulerImpl::sPrefChaoticScheduling = aPrefs[1] == '1';
   SchedulerImpl::sPrefPreemption = aPrefs[2] == '1';
   SchedulerImpl::sPrefUseMultipleQueues = aPrefs[3] == '1';
   MOZ_ASSERT(aPrefs[4] == ',');
   SchedulerImpl::sPrefThreadCount = atoi(aPrefs + 5);
 }