diff --git a/accessible/ipc/DocAccessibleChildBase.cpp b/accessible/ipc/DocAccessibleChildBase.cpp
index cd633234fdff..f36bd2c31423 100644
--- a/accessible/ipc/DocAccessibleChildBase.cpp
+++ b/accessible/ipc/DocAccessibleChildBase.cpp
@@ -82,29 +82,6 @@ DocAccessibleChildBase::SerializeTree(Accessible* aRoot,
   }
 }
 
-#if defined(XP_WIN)
-/* static */ void
-DocAccessibleChildBase::SetMsaaIds(Accessible* aRoot,
-                                   uint32_t& aMsaaIdIndex,
-                                   const nsTArray<MsaaMapping>& aNewMsaaIds)
-{
-  const MsaaMapping& mapping = aNewMsaaIds[aMsaaIdIndex];
-#if defined(DEBUG)
-  uint64_t id = reinterpret_cast<uint64_t>(aRoot->UniqueID());
-  MOZ_ASSERT(mapping.ID() == id);
-#endif // defined(DEBUG)
-  static_cast<AccessibleWrap*>(aRoot)->SetID(mapping.MsaaID());
-  ++aMsaaIdIndex;
-  if (aRoot->IsOuterDoc()) {
-    // This needs to match the tree traversal in SerializeTree
-    return;
-  }
-  for (uint32_t i = 0, n = aRoot->ChildCount(); i < n; ++i) {
-    SetMsaaIds(aRoot->GetChildAt(i), aMsaaIdIndex, aNewMsaaIds);
-  }
-}
-#endif // defined(XP_WIN)
-
 void
 DocAccessibleChildBase::ShowEvent(AccShowEvent* aShowEvent)
 {
@@ -114,22 +91,7 @@ DocAccessibleChildBase::ShowEvent(AccShowEvent* aShowEvent)
   nsTArray<AccessibleData> shownTree;
   ShowEventData data(parentID, idxInParent, shownTree);
   SerializeTree(aShowEvent->GetAccessible(), data.NewTree());
-#if defined(XP_WIN)
-  nsTArray<MsaaMapping> newMsaaIds;
-  SendShowEventInfo(data, &newMsaaIds);
-  // newMsaaIds could be empty if something went wrong in SendShowEvent()
-  if (!newMsaaIds.IsEmpty()) {
-    uint32_t index = 0;
-    SetMsaaIds(aShowEvent->GetAccessible(), index, newMsaaIds);
-  }
-  // NB: On Windows, SendShowEvent attaches the subtree and generates new IDs,
-  //     but does *NOT* fire the native event. We need to do that after
-  //     we've called SetMsaaIds.
-  SendEvent(reinterpret_cast<uint64_t>(aShowEvent->GetAccessible()->UniqueID()),
-            nsIAccessibleEvent::EVENT_SHOW);
-#else
   SendShowEvent(data, aShowEvent->IsFromUserInput());
-#endif // defined(XP_WIN)
 }
 
 } // namespace a11y
diff --git a/accessible/ipc/DocAccessibleChildBase.h b/accessible/ipc/DocAccessibleChildBase.h
index 4767b312f19a..2f1e4da9660f 100644
--- a/accessible/ipc/DocAccessibleChildBase.h
+++ b/accessible/ipc/DocAccessibleChildBase.h
@@ -60,10 +60,6 @@ public:
 protected:
   static uint32_t InterfacesFor(Accessible* aAcc);
   static void SerializeTree(Accessible* aRoot, nsTArray<AccessibleData>& aTree);
-#if defined(XP_WIN)
-  static void SetMsaaIds(Accessible* aRoot, uint32_t& aMsaaIdIndex,
-                         const nsTArray<MsaaMapping>& aNewMsaaIds);
-#endif
 
   DocAccessible*  mDoc;
 };
diff --git a/accessible/ipc/DocAccessibleParent.cpp b/accessible/ipc/DocAccessibleParent.cpp
index 8437d8183a2c..3c02bdea71ae 100644
--- a/accessible/ipc/DocAccessibleParent.cpp
+++ b/accessible/ipc/DocAccessibleParent.cpp
@@ -16,13 +16,8 @@ namespace mozilla {
 namespace a11y {
 
 bool
-#if defined(XP_WIN)
-DocAccessibleParent::RecvShowEventInfo(const ShowEventData& aData,
-                                       nsTArray<MsaaMapping>* aNewMsaaIds)
-#else
 DocAccessibleParent::RecvShowEvent(const ShowEventData& aData,
                                    const bool& aFromUser)
-#endif // defined(XP_WIN)
 {
   if (mShutdown)
     return true;
@@ -49,13 +44,7 @@ DocAccessibleParent::RecvShowEvent(const ShowEventData& aData,
     return true;
   }
 
-#if defined(XP_WIN)
-  aNewMsaaIds->SetCapacity(aData.NewTree().Length());
-  uint32_t consumed = AddSubtree(parent, aData.NewTree(), 0, newChildIdx,
-                                 aNewMsaaIds);
-#else
   uint32_t consumed = AddSubtree(parent, aData.NewTree(), 0, newChildIdx);
-#endif
   MOZ_ASSERT(consumed == aData.NewTree().Length());
 
   // XXX This shouldn't happen, but if we failed to add children then the below
@@ -73,9 +62,6 @@ DocAccessibleParent::RecvShowEvent(const ShowEventData& aData,
 
   MOZ_DIAGNOSTIC_ASSERT(CheckDocTree());
 
-  // NB: On Windows we dispatch the native event via a subsequent call to
-  // RecvEvent().
-#if !defined(XP_WIN)
   ProxyAccessible* target = parent->ChildAt(newChildIdx);
   ProxyShowHideEvent(target, parent, true, aFromUser);
 
@@ -90,7 +76,6 @@ DocAccessibleParent::RecvShowEvent(const ShowEventData& aData,
   RefPtr<xpcAccEvent> event = new xpcAccEvent(type, xpcAcc, doc, node,
                                               aFromUser);
   nsCoreUtils::DispatchAccEvent(Move(event));
-#endif
 
   return true;
 }
@@ -98,11 +83,7 @@ DocAccessibleParent::RecvShowEvent(const ShowEventData& aData,
 uint32_t
 DocAccessibleParent::AddSubtree(ProxyAccessible* aParent,
                                 const nsTArray<a11y::AccessibleData>& aNewTree,
-                                uint32_t aIdx, uint32_t aIdxInParent
-#if defined(XP_WIN)
-                                , nsTArray<MsaaMapping>* aNewMsaaIds
-#endif
-                                )
+                                uint32_t aIdx, uint32_t aIdxInParent)
 {
   if (aNewTree.Length() <= aIdx) {
     NS_ERROR("bad index in serialized tree!");
@@ -143,22 +124,10 @@ DocAccessibleParent::AddSubtree(ProxyAccessible* aParent,
   mAccessibles.PutEntry(newChild.ID())->mProxy = newProxy;
   ProxyCreated(newProxy, newChild.Interfaces());
 
-#if defined(XP_WIN)
-  Accessible* idForAcc = WrapperFor(newProxy);
-  MOZ_ASSERT(idForAcc);
-  uint32_t newMsaaId = AccessibleWrap::GetChildIDFor(idForAcc);
-  MOZ_ASSERT(newMsaaId);
-  aNewMsaaIds->AppendElement(MsaaMapping(newChild.ID(), newMsaaId));
-#endif // defined(XP_WIN)
-
   uint32_t accessibles = 1;
   uint32_t kids = newChild.ChildrenCount();
   for (uint32_t i = 0; i < kids; i++) {
-    uint32_t consumed = AddSubtree(newProxy, aNewTree, aIdx + accessibles, i
-#if defined(XP_WIN)
-                                   , aNewMsaaIds
-#endif
-                                  );
+    uint32_t consumed = AddSubtree(newProxy, aNewTree, aIdx + accessibles, i);
     if (!consumed)
       return 0;
 
@@ -507,8 +476,7 @@ DocAccessibleParent::GetXPCAccessible(ProxyAccessible* aProxy)
  */
 bool
 DocAccessibleParent::RecvCOMProxy(const IAccessibleHolder& aCOMProxy,
-                                  IAccessibleHolder* aParentCOMProxy,
-                                  uint32_t* aMsaaID)
+                                  IAccessibleHolder* aParentCOMProxy)
 {
   RefPtr<IAccessible> ptr(aCOMProxy.Get());
   SetCOMInterface(ptr);
@@ -520,8 +488,6 @@ DocAccessibleParent::RecvCOMProxy(const IAccessibleHolder& aCOMProxy,
   }
 
   aParentCOMProxy->Set(IAccessibleHolder::COMPtrType(rawNative));
-  Accessible* wrapper = WrapperFor(this);
-  *aMsaaID = AccessibleWrap::GetChildIDFor(wrapper);
   return true;
 }
 #endif // defined(XP_WIN)
diff --git a/accessible/ipc/DocAccessibleParent.h b/accessible/ipc/DocAccessibleParent.h
index 0a6377f78de6..94ff88cf49e0 100644
--- a/accessible/ipc/DocAccessibleParent.h
+++ b/accessible/ipc/DocAccessibleParent.h
@@ -50,13 +50,8 @@ public:
   virtual bool RecvEvent(const uint64_t& aID, const uint32_t& aType)
     override;
 
-#if defined(XP_WIN)
-  virtual bool RecvShowEventInfo(const ShowEventData& aData,
-                                 nsTArray<MsaaMapping>* aNewMsaaIds) override;
-#else
   virtual bool RecvShowEvent(const ShowEventData& aData, const bool& aFromUser)
     override;
-#endif // defined(XP_WIN)
   virtual bool RecvHideEvent(const uint64_t& aRootID, const bool& aFromUser)
     override;
   virtual bool RecvStateChangeEvent(const uint64_t& aID,
@@ -149,8 +144,7 @@ public:
 
 #if defined(XP_WIN)
   virtual bool RecvCOMProxy(const IAccessibleHolder& aCOMProxy,
-                            IAccessibleHolder* aParentCOMProxy,
-                            uint32_t* aMsaaID) override;
+                            IAccessibleHolder* aParentCOMProxy) override;
 #endif
 
 private:
@@ -180,11 +174,7 @@ private:
 
   uint32_t AddSubtree(ProxyAccessible* aParent,
                       const nsTArray<AccessibleData>& aNewTree, uint32_t aIdx,
-                      uint32_t aIdxInParent
-#if defined(XP_WIN)
-                      , nsTArray<MsaaMapping>* aNewMsaaIds
-#endif // defined(XP_WIN)
-                      );
+                      uint32_t aIdxInParent);
   MOZ_MUST_USE bool CheckDocTree() const;
   xpcAccessibleGeneric* GetXPCAccessible(ProxyAccessible* aProxy);
 
diff --git a/accessible/ipc/win/DocAccessibleChild.cpp b/accessible/ipc/win/DocAccessibleChild.cpp
index 5dbe9f78a26c..43a67bc09731 100644
--- a/accessible/ipc/win/DocAccessibleChild.cpp
+++ b/accessible/ipc/win/DocAccessibleChild.cpp
@@ -34,10 +34,8 @@ void
 DocAccessibleChild::SendCOMProxy(const IAccessibleHolder& aProxy)
 {
   IAccessibleHolder parentProxy;
-  uint32_t msaaID = AccessibleWrap::kNoID;
-  PDocAccessibleChild::SendCOMProxy(aProxy, &parentProxy, &msaaID);
+  PDocAccessibleChild::SendCOMProxy(aProxy, &parentProxy);
   mParentProxy.reset(parentProxy.Release());
-  mDoc->SetID(msaaID);
 }
 
 } // namespace a11y
diff --git a/accessible/ipc/win/PDocAccessible.ipdl b/accessible/ipc/win/PDocAccessible.ipdl
index 7aeb6635e11e..3af117dffa0f 100644
--- a/accessible/ipc/win/PDocAccessible.ipdl
+++ b/accessible/ipc/win/PDocAccessible.ipdl
@@ -27,12 +27,6 @@ struct ShowEventData
   AccessibleData[] NewTree;
 };
 
-struct MsaaMapping
-{
-  uint64_t ID;
-  uint32_t MsaaID;
-};
-
 struct Attribute
 {
   nsCString Name;
@@ -51,7 +45,7 @@ parent:
    * event.
    */
   async Event(uint64_t aID, uint32_t type);
-  sync ShowEventInfo(ShowEventData data) returns (MsaaMapping[] aNewMsaaIds);
+  async ShowEvent(ShowEventData data, bool aFromuser);
   async HideEvent(uint64_t aRootID, bool aFromUser);
   async StateChangeEvent(uint64_t aID, uint64_t aState, bool aEnabled);
   async CaretMoveEvent(uint64_t aID, int32_t aOffset);
@@ -69,7 +63,7 @@ parent:
   // For now we'll add the command to send the proxy here. This might move to
   // PDocAccessible constructor in PBrowser.
   sync COMProxy(IAccessibleHolder aDocCOMProxy)
-    returns(IAccessibleHolder aParentCOMProxy, uint32_t aMsaaID);
+    returns(IAccessibleHolder aParentCOMProxy);
 
 child:
   async __delete__();
diff --git a/accessible/windows/msaa/AccessibleWrap.cpp b/accessible/windows/msaa/AccessibleWrap.cpp
index d8274f01adee..0d8b85a66763 100644
--- a/accessible/windows/msaa/AccessibleWrap.cpp
+++ b/accessible/windows/msaa/AccessibleWrap.cpp
@@ -9,7 +9,6 @@
 
 #include "Compatibility.h"
 #include "DocAccessible-inl.h"
-#include "mozilla/a11y/DocAccessibleChild.h"
 #include "mozilla/a11y/DocAccessibleParent.h"
 #include "EnumVariant.h"
 #include "nsAccUtils.h"
@@ -70,7 +69,9 @@ static const int32_t kIEnumVariantDisconnected = -1;
 ////////////////////////////////////////////////////////////////////////////////
 AccessibleWrap::AccessibleWrap(nsIContent* aContent, DocAccessible* aDoc) :
   Accessible(aContent, aDoc)
+#ifdef _WIN64
   , mID(kNoID)
+#endif
 {
 }
 
@@ -89,6 +90,7 @@ NS_IMPL_ISUPPORTS_INHERITED0(AccessibleWrap, Accessible)
 void
 AccessibleWrap::Shutdown()
 {
+#ifdef _WIN64
   if (mID != kNoID) {
     auto doc = static_cast<DocAccessibleWrap*>(mDoc);
     MOZ_ASSERT(doc);
@@ -96,6 +98,7 @@ AccessibleWrap::Shutdown()
       doc->RemoveID(mID);
     }
   }
+#endif
 
   Accessible::Shutdown();
 }
@@ -1144,22 +1147,9 @@ AccessibleWrap::GetNativeInterface(void** aOutAccessible)
   NS_ADDREF_THIS();
 }
 
-void
-AccessibleWrap::SetID(uint32_t aID)
-{
-  MOZ_ASSERT(XRE_IsContentProcess());
-  mID = aID;
-  DocAccessibleWrap* doc = static_cast<DocAccessibleWrap*>(Document());
-  DebugOnly<AccessibleWrap*> checkAcc = nullptr;
-  MOZ_ASSERT(!(checkAcc = doc->GetAccessibleByID(aID)) ||
-             checkAcc->GetExistingID() == aID);
-  doc->AddID(aID, this);
-}
-
 void
 AccessibleWrap::FireWinEvent(Accessible* aTarget, uint32_t aEventType)
 {
-  MOZ_ASSERT(XRE_IsParentProcess());
   static_assert(sizeof(gWinEventMap)/sizeof(gWinEventMap[0]) == nsIAccessibleEvent::EVENT_LAST_ENTRY,
                 "MSAA event map skewed");
 
@@ -1256,13 +1246,6 @@ AccessibleWrap::GetChildIDFor(Accessible* aAccessible)
     return 0;
   }
 
-  // Content should use mID which has been generated by the chrome process.
-  if (XRE_IsContentProcess() && !aAccessible->IsApplication()) {
-    const uint32_t id = static_cast<AccessibleWrap*>(aAccessible)->mID;
-    MOZ_ASSERT(id != kNoID);
-    return id;
-  }
-
 #ifdef _WIN64
   if (!aAccessible->Document() && !aAccessible->IsProxy())
     return 0;
@@ -1362,6 +1345,7 @@ AccessibleWrap::NativeAccessible(Accessible* aAccessible)
   return static_cast<IDispatch*>(msaaAccessible);
 }
 
+#ifdef _WIN64
 static Accessible*
 GetAccessibleInSubtree(DocAccessible* aDoc, uint32_t aID)
 {
@@ -1378,6 +1362,7 @@ GetAccessibleInSubtree(DocAccessible* aDoc, uint32_t aID)
 
     return nullptr;
   }
+#endif
 
 static AccessibleWrap*
 GetProxiedAccessibleInSubtree(const DocAccessibleParent* aDoc, uint32_t aID)
@@ -1441,9 +1426,7 @@ AccessibleWrap::GetXPAccessibleFor(const VARIANT& aVarChild)
 #ifdef _WIN64
       GetAccessibleInSubtree(document, static_cast<uint32_t>(aVarChild.lVal));
 #else
-      XRE_IsContentProcess() ?
-        GetAccessibleInSubtree(document, static_cast<uint32_t>(aVarChild.lVal)) :
-        document->GetAccessibleByUniqueIDInSubtree(uniqueID);
+      document->GetAccessibleByUniqueIDInSubtree(uniqueID);
 #endif
 
     // If it is a document then just return an accessible.
diff --git a/accessible/windows/msaa/AccessibleWrap.h b/accessible/windows/msaa/AccessibleWrap.h
index bd50c98a34df..26a94f69463f 100644
--- a/accessible/windows/msaa/AccessibleWrap.h
+++ b/accessible/windows/msaa/AccessibleWrap.h
@@ -179,15 +179,17 @@ public: // construction, destruction
 
   static IDispatch* NativeAccessible(Accessible* aAccessible);
 
+#ifdef _WIN64
   uint32_t GetExistingID() const { return mID; }
   static const uint32_t kNoID = 0;
-  // This is only valid to call in content
-  void SetID(uint32_t aID);
+#endif
 
 protected:
   virtual ~AccessibleWrap();
 
+#ifdef _WIN64
   uint32_t mID;
+#endif
 
   /**
    * Return the wrapper for the document's proxy.
diff --git a/accessible/windows/msaa/DocAccessibleWrap.cpp b/accessible/windows/msaa/DocAccessibleWrap.cpp
index 2e022b851e0d..fa6cc155fcc2 100644
--- a/accessible/windows/msaa/DocAccessibleWrap.cpp
+++ b/accessible/windows/msaa/DocAccessibleWrap.cpp
@@ -48,10 +48,15 @@ STDMETHODIMP
 DocAccessibleWrap::get_accParent(
       /* [retval][out] */ IDispatch __RPC_FAR *__RPC_FAR *ppdispParent)
 {
+  HRESULT hr = DocAccessible::get_accParent(ppdispParent);
+  if (*ppdispParent) {
+    return hr;
+  }
+
   // We might be a top-level document in a content process.
   DocAccessibleChild* ipcDoc = IPCDoc();
   if (!ipcDoc) {
-    return DocAccessible::get_accParent(ppdispParent);
+    return S_FALSE;
   }
   IAccessible* dispParent = ipcDoc->GetParentIAccessible();
   if (!dispParent) {
diff --git a/accessible/windows/msaa/DocAccessibleWrap.h b/accessible/windows/msaa/DocAccessibleWrap.h
index effa23848cd8..90ae3d052725 100644
--- a/accessible/windows/msaa/DocAccessibleWrap.h
+++ b/accessible/windows/msaa/DocAccessibleWrap.h
@@ -40,11 +40,13 @@ public:
   /**
    * Manage the mapping from id to Accessible.
    */
+#ifdef _WIN64
   void AddID(uint32_t aID, AccessibleWrap* aAcc)
     { mIDToAccessibleMap.Put(aID, aAcc); }
   void RemoveID(uint32_t aID) { mIDToAccessibleMap.Remove(aID); }
   AccessibleWrap* GetAccessibleByID(uint32_t aID) const
     { return mIDToAccessibleMap.Get(aID); }
+#endif
 
 protected:
   // DocAccessible
@@ -56,7 +58,9 @@ protected:
   /*
    * This provides a mapping from 32 bit id to accessible objects.
    */
+#ifdef _WIN64
   nsDataHashtable<nsUint32HashKey, AccessibleWrap*> mIDToAccessibleMap;
+#endif
 };
 
 } // namespace a11y