--- a/dom/base/nsDOMClassInfo.cpp
+++ b/dom/base/nsDOMClassInfo.cpp
@@ -261,68 +261,16 @@ CutPrefix(const char *aName) {
 
   if (strncmp(aName, prefix_nsI, sizeof(prefix_nsI) - 1) == 0) {
     return aName + sizeof(prefix_nsI) - 1;
   }
 
   return aName;
 }
 
-// static
-nsresult
-nsDOMClassInfo::RegisterClassProtos(int32_t aClassInfoID)
-{
-  nsScriptNameSpaceManager *nameSpaceManager = GetNameSpaceManager();
-  NS_ENSURE_TRUE(nameSpaceManager, NS_ERROR_NOT_INITIALIZED);
-  bool found_old;
-
-  const nsIID *primary_iid = sClassInfoData[aClassInfoID].mProtoChainInterface;
-
-  if (!primary_iid || primary_iid == &NS_GET_IID(nsISupports)) {
-    return NS_OK;
-  }
-
-  nsCOMPtr<nsIInterfaceInfoManager>
-    iim(do_GetService(NS_INTERFACEINFOMANAGER_SERVICE_CONTRACTID));
-  NS_ENSURE_TRUE(iim, NS_ERROR_NOT_AVAILABLE);
-
-  nsCOMPtr<nsIInterfaceInfo> if_info;
-  bool first = true;
-
-  iim->GetInfoForIID(primary_iid, getter_AddRefs(if_info));
-
-  while (if_info) {
-    const nsIID *iid = nullptr;
-
-    if_info->GetIIDShared(&iid);
-    NS_ENSURE_TRUE(iid, NS_ERROR_UNEXPECTED);
-
-    if (iid->Equals(NS_GET_IID(nsISupports))) {
-      break;
-    }
-
-    const char *name = nullptr;
-    if_info->GetNameShared(&name);
-    NS_ENSURE_TRUE(name, NS_ERROR_UNEXPECTED);
-
-    nameSpaceManager->RegisterClassProto(CutPrefix(name), iid, &found_old);
-
-    if (first) {
-      first = false;
-    } else if (found_old) {
-      break;
-    }
-
-    nsCOMPtr<nsIInterfaceInfo> tmp(if_info);
-    tmp->GetParent(getter_AddRefs(if_info));
-  }
-
-  return NS_OK;
-}
-
 #define _DOM_CLASSINFO_MAP_BEGIN(_class, _ifptr, _has_class_if)               \
   {                                                                           \
     nsDOMClassInfoData &d = sClassInfoData[eDOMClassInfo_##_class##_id];      \
     d.mProtoChainInterface = _ifptr;                                          \
     d.mHasClassInterface = _has_class_if;                                     \
     static const nsIID *interface_list[] = {
 
 #define DOM_CLASSINFO_MAP_BEGIN(_class, _interface)                           \
@@ -419,20 +367,16 @@ nsDOMClassInfo::Init()
     if (i == eDOMClassInfo_DOMPrototype_id) {
       continue;
     }
 
     nsDOMClassInfoData& data = sClassInfoData[i];
     nameSpaceManager->RegisterClassName(data.mClass.name, i, &data.mNameUTF16);
   }
 
-  for (i = 0; i < eDOMClassInfoIDCount; ++i) {
-    RegisterClassProtos(i);
-  }
-
   sIsInitialized = true;
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDOMClassInfo::GetInterfaces(uint32_t *aCount, nsIID ***aArray)
 {

--- a/dom/base/nsDOMClassInfo.h
+++ b/dom/base/nsDOMClassInfo.h
@@ -95,18 +95,16 @@ protected:
   friend nsIClassInfo* NS_GetDOMClassInfoInstance(nsDOMClassInfoID aID);
 
   const nsDOMClassInfoData* mData;
 
   virtual void PreserveWrapper(nsISupports *aNative) override
   {
   }
 
-  static nsresult RegisterClassProtos(int32_t aDOMClassInfoID);
-
   static nsIXPConnect *sXPConnect;
 
   // nsIXPCScriptable code
   static nsresult DefineStaticJSVals();
 
   static bool sIsInitialized;
 
 public:

--- a/dom/base/nsScriptNameSpaceManager.cpp
+++ b/dom/base/nsScriptNameSpaceManager.cpp
@@ -222,40 +222,16 @@ nsScriptNameSpaceManager::RegisterClassN
 
   s->mType = nsGlobalNameStruct::eTypeClassConstructor;
   s->mDOMClassInfoID = aDOMClassInfoID;
 
   return NS_OK;
 }
 
 nsresult
-nsScriptNameSpaceManager::RegisterClassProto(const char *aClassName,
-                                             const nsIID *aConstructorProtoIID,
-                                             bool *aFoundOld)
-{
-  NS_ENSURE_ARG_POINTER(aConstructorProtoIID);
-
-  *aFoundOld = false;
-
-  nsGlobalNameStruct *s = AddToHash(aClassName);
-  NS_ENSURE_TRUE(s, NS_ERROR_OUT_OF_MEMORY);
-
-  if (s->mType != nsGlobalNameStruct::eTypeNotInitialized) {
-    *aFoundOld = true;
-
-    return NS_OK;
-  }
-
-  s->mType = nsGlobalNameStruct::eTypeClassProto;
-  s->mIID = *aConstructorProtoIID;
-
-  return NS_OK;
-}
-
-nsresult
 nsScriptNameSpaceManager::OperateCategoryEntryHash(nsICategoryManager* aCategoryManager,
                                                    const char* aCategory,
                                                    nsISupports* aEntry,
                                                    bool aRemove)
 {
   MOZ_ASSERT(aCategoryManager);
   // Get the type from the category name.
   // NOTE: we could have passed the type in FillHash() and guessed it in

--- a/dom/base/nsScriptNameSpaceManager.h
+++ b/dom/base/nsScriptNameSpaceManager.h
@@ -72,20 +72,16 @@ public:
   // in the nsGlobalNameStruct.
   const nsGlobalNameStruct* LookupName(const nsAString& aName,
                                        const char16_t **aClassName = nullptr);
 
   nsresult RegisterClassName(const char *aClassName,
                              int32_t aDOMClassInfoID,
                              const char16_t **aResult);
 
-  nsresult RegisterClassProto(const char *aClassName,
-                              const nsIID *aConstructorProtoIID,
-                              bool *aFoundOld);
-
   class NameIterator : public PLDHashTable::Iterator
   {
   public:
     typedef PLDHashTable::Iterator Base;
     explicit NameIterator(PLDHashTable* aTable) : Base(aTable) {}
     NameIterator(NameIterator&& aOther) : Base(mozilla::Move(aOther.mTable)) {}
 
     const GlobalNameMapEntry* Get() const