diff --git a/dom/workers/RuntimeService.cpp b/dom/workers/RuntimeService.cpp
index b67ecbb5e..c10acd855 100644
--- a/dom/workers/RuntimeService.cpp
+++ b/dom/workers/RuntimeService.cpp
@@ -1636,6 +1636,8 @@ RuntimeService::RegisterWorker(WorkerPrivate* aWorkerPrivate)
   }
   else {
     if (!mNavigatorPropertiesLoaded) {
+      MutexAutoLock lock(mMutex);
+      
       Navigator::AppName(mNavigatorProperties.mAppName,
                          false /* aUsePrefOverriddenValue */);
       if (NS_FAILED(Navigator::GetAppVersion(mNavigatorProperties.mAppVersion,
@@ -2564,6 +2566,7 @@ void
 RuntimeService::UpdateAppVersionOverridePreference(const nsAString& aValue)
 {
   AssertIsOnMainThread();
+  MutexAutoLock lock(mMutex);
   mNavigatorProperties.mAppVersionOverridden = aValue;
 }
 
@@ -2571,6 +2574,7 @@ void
 RuntimeService::UpdatePlatformOverridePreference(const nsAString& aValue)
 {
   AssertIsOnMainThread();
+  MutexAutoLock lock(mMutex);
   mNavigatorProperties.mPlatformOverridden = aValue;
 }
 
@@ -2585,7 +2589,10 @@ RuntimeService::UpdateAllWorkerLanguages(const nsTArray<nsString>& aLanguages)
 {
   MOZ_ASSERT(NS_IsMainThread());
 
-  mNavigatorProperties.mLanguages = aLanguages;
+  { 
+    MutexAutoLock lock(mMutex);
+    mNavigatorProperties.mLanguages = aLanguages;
+  }
   BROADCAST_ALL_WORKERS(UpdateLanguages, aLanguages);
 }
 
diff --git a/dom/workers/RuntimeService.h b/dom/workers/RuntimeService.h
index 2ab8cbabe..946a0200d 100644
--- a/dom/workers/RuntimeService.h
+++ b/dom/workers/RuntimeService.h
@@ -75,7 +75,7 @@ class RuntimeService final : public nsIObserver
 
   struct IdleThreadInfo;
 
-  mozilla::Mutex mMutex;
+  mutable mozilla::Mutex mMutex;
 
   // Protected by mMutex.
   nsClassHashtable<nsCStringHashKey, WorkerDomainInfo> mDomainMap;
@@ -157,9 +157,10 @@ public:
   void
   ForgetSharedWorker(WorkerPrivate* aWorkerPrivate);
 
-  const NavigatorProperties&
+  NavigatorProperties
   GetNavigatorProperties() const
   {
+    MutexAutoLock lock(mMutex);
     return mNavigatorProperties;
   }
 
diff --git a/dom/workers/WorkerNavigator.cpp b/dom/workers/WorkerNavigator.cpp
index aaeb58703..7c6395959 100644
--- a/dom/workers/WorkerNavigator.cpp
+++ b/dom/workers/WorkerNavigator.cpp
@@ -51,7 +51,7 @@ WorkerNavigator::Create(bool aOnLine)
   RuntimeService* rts = RuntimeService::GetService();
   MOZ_ASSERT(rts);
 
-  const RuntimeService::NavigatorProperties& properties =
+  RuntimeService::NavigatorProperties properties =
     rts->GetNavigatorProperties();
 
   RefPtr<WorkerNavigator> navigator =