diff --git a/dom/storage/DOMStorageCache.cpp b/dom/storage/DOMStorageCache.cpp
index c2f049569b2..4c84ac0dcbb 100644
--- a/dom/storage/DOMStorageCache.cpp
+++ b/dom/storage/DOMStorageCache.cpp
@@ -564,9 +564,16 @@ DOMStorageCache::Clear(const DOMStorage* aStorage)
 void
 DOMStorageCache::CloneFrom(const DOMStorageCache* aThat)
 {
-  mLoaded = aThat->mLoaded;
+  // This will never be called on anything else than SessionStorage.
+  // This means mData will never be touched on any other thread than
+  // the main thread and it never went through the loading process.
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!mPersistent);
+  MOZ_ASSERT(!(bool)aThat->mLoaded);
+
+  mLoaded = false;
   mInitialized = aThat->mInitialized;
-  mPersistent = aThat->mPersistent;
+  mPersistent = false;
   mSessionOnlyDataSetActive = aThat->mSessionOnlyDataSetActive;
 
   for (uint32_t i = 0; i < kDataSetCount; ++i) {
diff --git a/dom/storage/DOMStorageCache.h b/dom/storage/DOMStorageCache.h
index 3ee3ce1e5c7..2bd4737f45b 100644
--- a/dom/storage/DOMStorageCache.h
+++ b/dom/storage/DOMStorageCache.h
@@ -15,6 +15,7 @@
 #include "nsHashKeys.h"
 #include "mozilla/Monitor.h"
 #include "mozilla/Telemetry.h"
+#include "mozilla/Atomics.h"
 #include "nsAutoPtr.h"
 
 namespace mozilla {
@@ -220,8 +221,9 @@ private:
   // Flag that is initially false.  When the cache is about to work with
   // the database (i.e. it is persistent) this flags is set to true after
   // all keys and coresponding values are loaded from the database.
-  // This flag never goes from true back to false.
-  bool mLoaded;
+  // This flag never goes from true back to false.  Since this flag is
+  // critical for mData hashtable synchronization, it's made atomic.
+  Atomic<bool, ReleaseAcquire> mLoaded;
 
   // Result of load from the database.  Valid after mLoaded flag has been set.
   nsresult mLoadResult;