--- a/xpcom/glue/nsTHashtable.h
+++ b/xpcom/glue/nsTHashtable.h
@@ -5,16 +5,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef nsTHashtable_h__
 #define nsTHashtable_h__
 
 #include "nscore.h"
 #include "pldhash.h"
 #include "nsDebug.h"
+#include "mozilla/Assertions.h"
 #include "mozilla/MemoryChecking.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/Move.h"
 #include "mozilla/fallible.h"
 #include "mozilla/PodOperations.h"
 
 #include <new>
 
@@ -284,16 +285,26 @@ public:
    * If the EntryType defines SizeOfExcludingThis, there's no need to define a new
    * SizeOfEntryExcludingThisFun.
    */
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
   {
     return SizeOfIncludingThis(BasicSizeOfEntryExcludingThisFun, aMallocSizeOf);
   }
 
+  /**
+   * Swap the elements in this hashtable with the elements in aOther.
+   */
+  void SwapElements(nsTHashtable<EntryType>& aOther)
+  {
+    MOZ_ASSERT_IF(this->mTable.ops && aOther.mTable.ops,
+                  this->mTable.ops == aOther.mTable.ops);
+    mozilla::Swap(this->mTable, aOther.mTable);
+  }
+
 #ifdef DEBUG
   /**
    * Mark the table as constant after initialization.
    *
    * This will prevent assertions when a read-only hash is accessed on multiple
    * threads without synchronization.
    */
   void MarkImmutable()