--- a/dom/xbl/nsXBLPrototypeBinding.cpp
+++ b/dom/xbl/nsXBLPrototypeBinding.cpp
@@ -1712,16 +1712,18 @@ nsXBLPrototypeBinding::AppendStyleSheets
     mResources->AppendStyleSheetsTo(aResult);
   }
 }
 
 size_t
 nsXBLPrototypeBinding::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   size_t n = aMallocSizeOf(this);
+  n += mPrototypeHandler
+    ? mPrototypeHandler->SizeOfIncludingThis(aMallocSizeOf) : 0;
   n += mResources ? mResources->SizeOfIncludingThis(aMallocSizeOf) : 0;
 
   if (mAttributeTable) {
     n += mAttributeTable->ShallowSizeOfIncludingThis(aMallocSizeOf);
     for (auto iter = mAttributeTable->Iter(); !iter.Done(); iter.Next()) {
       InnerAttributeTable* table = iter.UserData();
       n += table->ShallowSizeOfIncludingThis(aMallocSizeOf);
       for (auto iter2 = table->Iter(); !iter2.Done(); iter2.Next()) {
@@ -1732,14 +1734,13 @@ nsXBLPrototypeBinding::SizeOfIncludingTh
 
   n += mInterfaceTable.ShallowSizeOfExcludingThis(aMallocSizeOf);
   n += mKeyHandlers.ShallowSizeOfExcludingThis(aMallocSizeOf);
 
   // Measurement of the following members may be added later if DMD finds it
   // is worthwhile:
   // - mBindingURI
   // - mAlternateBindingURI
-  // - mPrototypeHandler
   // - mBaseBindingURI
   // - mImplementation
 
   return n;
 }

--- a/dom/xbl/nsXBLPrototypeHandler.cpp
+++ b/dom/xbl/nsXBLPrototypeHandler.cpp
@@ -1130,8 +1130,23 @@ nsXBLPrototypeHandler::Write(nsIObjectOu
 
   rv = aStream->WriteWStringZ(nsDependentAtomString(mEventName).get());
   NS_ENSURE_SUCCESS(rv, rv);
 
   rv = aStream->Write32(mLineNumber);
   NS_ENSURE_SUCCESS(rv, rv);
   return aStream->WriteWStringZ(mHandlerText ? mHandlerText : u"");
 }
+
+size_t
+nsXBLPrototypeHandler::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
+{
+  size_t n = 0;
+  for (const nsXBLPrototypeHandler* handler = this;
+       handler; handler = handler->mNextHandler) {
+    n += aMallocSizeOf(handler);
+    if (!(mType & NS_HANDLER_TYPE_XUL)) {
+      n += aMallocSizeOf(handler->mHandlerText);
+    }
+    n += mHandler ? aMallocSizeOf(handler->mHandler) : 0;
+  }
+  return n;
+}

--- a/dom/xbl/nsXBLPrototypeHandler.h
+++ b/dom/xbl/nsXBLPrototypeHandler.h
@@ -3,16 +3,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef nsXBLPrototypeHandler_h__
 #define nsXBLPrototypeHandler_h__
 
 #include "mozilla/EventForwards.h"
+#include "mozilla/MemoryReporting.h"
 #include "nsAtom.h"
 #include "nsString.h"
 #include "nsCOMPtr.h"
 #include "nsIController.h"
 #include "nsAutoPtr.h"
 #include "nsXBLEventHandler.h"
 #include "nsIWeakReference.h"
 #include "nsCycleCollectionParticipant.h"
@@ -167,16 +168,18 @@ public:
   bool AllowUntrustedEvents()
   {
     return (mType & NS_HANDLER_ALLOW_UNTRUSTED) != 0;
   }
 
   nsresult Read(nsIObjectInputStream* aStream);
   nsresult Write(nsIObjectOutputStream* aStream);
 
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
 public:
   static uint32_t gRefCnt;
 
 protected:
   void Init() {
     ++gRefCnt;
     if (gRefCnt == 1)
       // Get the primary accelerator key.

--- a/dom/xbl/nsXBLPrototypeResources.cpp
+++ b/dom/xbl/nsXBLPrototypeResources.cpp
@@ -240,16 +240,19 @@ nsXBLPrototypeResources::AppendStyleShee
 MOZ_DEFINE_MALLOC_SIZE_OF(ServoAuthorStylesMallocSizeOf)
 MOZ_DEFINE_MALLOC_ENCLOSING_SIZE_OF(ServoAuthorStylesMallocEnclosingSizeOf)
 
 size_t
 nsXBLPrototypeResources::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   size_t n = aMallocSizeOf(this);
   n += mStyleSheetList.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  for (const auto& sheet : mStyleSheetList) {
+    n += sheet->SizeOfIncludingThis(aMallocSizeOf);
+  }
 #ifdef MOZ_OLD_STYLE
   n += mRuleProcessor ? mRuleProcessor->SizeOfIncludingThis(aMallocSizeOf) : 0;
 #endif
   n += mServoStyles ? Servo_AuthorStyles_SizeOfIncludingThis(
       ServoAuthorStylesMallocSizeOf,
       ServoAuthorStylesMallocEnclosingSizeOf,
       mServoStyles.get()) : 0;
   n += mStyleRuleMap ? mStyleRuleMap->SizeOfIncludingThis(aMallocSizeOf) : 0;