diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 588dd0472f00..86ba1ffa1be9 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -210,6 +210,7 @@
 #include "mozilla/dom/WindowBinding.h"
 #include "nsITabChild.h"
 #include "nsIDOMMediaQueryList.h"
+#include "mozilla/dom/DOMJSClass.h"
 
 #ifdef MOZ_WEBSPEECH
 #include "mozilla/dom/SpeechSynthesis.h"
@@ -12658,6 +12659,12 @@ nsGlobalWindow::AddSizeOfIncludingThis(nsWindowSizes* aWindowSizes) const
     const_cast<nsTHashtable<nsPtrHashKey<nsDOMEventTargetHelper> >*>
       (&mEventTargetObjects)->EnumerateEntries(CollectSizeAndListenerCount,
                                                aWindowSizes);
+
+  JSObject* global = FastGetGlobalJSObject();
+  if (IsInnerWindow() && global) {
+    JS::Heap<JSObject*>* cache = GetProtoAndIfaceArray(global);
+    aWindowSizes->mProtoIfaceCacheSize += aWindowSizes->mMallocSizeOf(cache);
+  }
 }
 
 
diff --git a/dom/base/nsGlobalWindow.h b/dom/base/nsGlobalWindow.h
index 39923deaec40..2924c80f5864 100644
--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -342,7 +342,7 @@ public:
 
   // nsIScriptGlobalObject
   virtual nsIScriptContext *GetContext();
-  JSObject *FastGetGlobalJSObject()
+  JSObject *FastGetGlobalJSObject() const
   {
     return mJSObject;
   }