diff --git a/js/xpconnect/src/XPCJSRuntime.cpp b/js/xpconnect/src/XPCJSRuntime.cpp
index 6bc2609212fa..50a48c112878 100644
--- a/js/xpconnect/src/XPCJSRuntime.cpp
+++ b/js/xpconnect/src/XPCJSRuntime.cpp
@@ -525,18 +525,11 @@ XPCJSRuntime::TraverseAdditionalNativeRoots(nsCycleCollectionNoteRootCallback &c
     }
 }
 
-static PLDHashOperator
-UnmarkJSHolder(void *holder, nsScriptObjectTracer *&tracer, void *arg)
-{
-    tracer->CanSkip(holder, true);
-    return PL_DHASH_NEXT;
-}
-
 void
 XPCJSRuntime::UnmarkSkippableJSHolders()
 {
     XPCAutoLock lock(mMapLock);
-    mJSHolders.Enumerate(UnmarkJSHolder, nullptr);
+    CycleCollectedJSRuntime::UnmarkSkippableJSHolders();
 }
 
 void
@@ -1113,9 +1106,7 @@ XPCJSRuntime::SizeOfIncludingThis(nsMallocSizeOfFun mallocSizeOf)
     n += mClassInfo2NativeSetMap->ShallowSizeOfIncludingThis(mallocSizeOf);
     n += mNativeSetMap->SizeOfIncludingThis(mallocSizeOf);
 
-    // NULL for the second arg;  we're not measuring anything hanging off the
-    // entries in mJSHolders.
-    n += mJSHolders.SizeOfExcludingThis(nullptr, mallocSizeOf);
+    n += CycleCollectedJSRuntime::SizeOfExcludingThis(mallocSizeOf);
 
     // There are other XPCJSRuntime members that could be measured; the above
     // ones have been seen by DMD to be worth measuring.  More stuff may be
diff --git a/xpcom/base/CycleCollectedJSRuntime.cpp b/xpcom/base/CycleCollectedJSRuntime.cpp
index 9f93ce193350..d914221c3aad 100644
--- a/xpcom/base/CycleCollectedJSRuntime.cpp
+++ b/xpcom/base/CycleCollectedJSRuntime.cpp
@@ -479,6 +479,31 @@ CycleCollectedJSRuntime::~CycleCollectedJSRuntime()
   mJSRuntime = nullptr;
 }
 
+size_t
+CycleCollectedJSRuntime::SizeOfExcludingThis(nsMallocSizeOfFun aMallocSizeOf) const
+{
+  size_t n = 0;
+
+  // NULL for the second arg;  we're not measuring anything hanging off the
+  // entries in mJSHolders.
+  n += mJSHolders.SizeOfExcludingThis(nullptr, aMallocSizeOf);
+
+  return n;
+}
+
+static PLDHashOperator
+UnmarkJSHolder(void* holder, nsScriptObjectTracer*& tracer, void* arg)
+{
+  tracer->CanSkip(holder, true);
+  return PL_DHASH_NEXT;
+}
+
+void
+CycleCollectedJSRuntime::UnmarkSkippableJSHolders()
+{
+  mJSHolders.Enumerate(UnmarkJSHolder, nullptr);
+}
+
 void
 CycleCollectedJSRuntime::MaybeTraceGlobals(JSTracer* aTracer) const
 {
diff --git a/xpcom/base/CycleCollectedJSRuntime.h b/xpcom/base/CycleCollectedJSRuntime.h
index e6c19a673d09..b5230ff3ada9 100644
--- a/xpcom/base/CycleCollectedJSRuntime.h
+++ b/xpcom/base/CycleCollectedJSRuntime.h
@@ -88,6 +88,9 @@ protected:
     return mJSRuntime;
   }
 
+  size_t SizeOfExcludingThis(nsMallocSizeOfFun aMallocSizeOf) const;
+  void UnmarkSkippableJSHolders();
+
   virtual void TraverseAdditionalNativeRoots(nsCycleCollectionNoteRootCallback& aCb) = 0;
   virtual void TraceAdditionalNativeGrayRoots(JSTracer* aTracer) = 0;
 
@@ -168,10 +171,6 @@ public:
   bool NeedCollect() const;
   void Collect(uint32_t reason) const;
 
-// XXXkhuey should be private
-protected:
-  nsDataHashtable<nsPtrHashKey<void>, nsScriptObjectTracer*> mJSHolders;
-
 private:
   typedef const CCParticipantVTable<JSGCThingParticipant>::Type GCThingParticipantVTable;
   const GCThingParticipantVTable mGCThingCycleCollectorGlobal;
@@ -181,6 +180,8 @@ private:
 
   JSRuntime* mJSRuntime;
 
+  nsDataHashtable<nsPtrHashKey<void>, nsScriptObjectTracer*> mJSHolders;
+
 #ifdef DEBUG
   void* mObjectToUnlink;
   bool mExpectUnrootedGlobals;