diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 00288e370c1b..38044f825d01 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -6034,15 +6034,9 @@ nsContentUtils::SetDataTransferInEvent(WidgetDragEvent* aDragEvent)
   nsCOMPtr<nsIDragSession> dragSession = GetDragSession();
   NS_ENSURE_TRUE(dragSession, NS_OK); // no drag in progress
 
-  nsCOMPtr<nsIDOMDataTransfer> dataTransfer;
-  nsCOMPtr<DataTransfer> initialDataTransfer;
-  dragSession->GetDataTransfer(getter_AddRefs(dataTransfer));
-  if (dataTransfer) {
-    initialDataTransfer = do_QueryInterface(dataTransfer);
-    if (!initialDataTransfer) {
-      return NS_ERROR_FAILURE;
-    }
-  } else {
+  RefPtr<DataTransfer> initialDataTransfer =
+    dragSession->GetDataTransfer();
+  if (!initialDataTransfer) {
     // A dataTransfer won't exist when a drag was started by some other
     // means, for instance calling the drag service directly, or a drag
     // from another application. In either case, a new dataTransfer should
diff --git a/dom/events/DataTransfer.cpp b/dom/events/DataTransfer.cpp
index cce3b3355c33..dedcafc7bef0 100644
--- a/dom/events/DataTransfer.cpp
+++ b/dom/events/DataTransfer.cpp
@@ -483,22 +483,11 @@ DataTransfer::ClearData(const Optional<nsAString>& aFormat,
   }
 }
 
-NS_IMETHODIMP
-DataTransfer::GetMozCursor(nsAString& aCursorState)
-{
-  nsString cursor;
-  GetMozCursor(cursor);
-  aCursorState = cursor;
-  return NS_OK;
-}
-
-NS_IMETHODIMP
+void
 DataTransfer::SetMozCursor(const nsAString& aCursorState)
 {
   // Lock the cursor to an arrow during the drag.
   mCursorState = aCursorState.EqualsLiteral("default");
-
-  return NS_OK;
 }
 
 already_AddRefed<nsINode>
diff --git a/dom/events/DataTransfer.h b/dom/events/DataTransfer.h
index 62ffcf6bd66d..1d4bb346338c 100644
--- a/dom/events/DataTransfer.h
+++ b/dom/events/DataTransfer.h
@@ -205,7 +205,7 @@ public:
 
   uint32_t MozItemCount() const;
 
-  void GetMozCursor(nsString& aCursor)
+  void GetMozCursor(nsAString& aCursor)
   {
     if (mCursorState) {
       aCursor.AssignLiteral("default");
@@ -213,6 +213,7 @@ public:
       aCursor.AssignLiteral("auto");
     }
   }
+  void SetMozCursor(const nsAString& aCursor);
 
   already_AddRefed<DOMStringList> MozTypesAt(uint32_t aIndex,
                                              CallerType aCallerType,
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index 80ff5e806b05..1271d61bd361 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -1337,8 +1337,7 @@ EventStateManager::DispatchCrossProcessEvent(WidgetEvent* aEvent,
       dragSession->DragEventDispatchedToChildProcess();
       dragSession->GetDragAction(&action);
       dragSession->GetTriggeringPrincipalURISpec(principalURISpec);
-      nsCOMPtr<nsIDOMDataTransfer> initialDataTransfer;
-      dragSession->GetDataTransfer(getter_AddRefs(initialDataTransfer));
+      RefPtr<DataTransfer> initialDataTransfer = dragSession->GetDataTransfer();
       if (initialDataTransfer) {
         initialDataTransfer->GetDropEffectInt(&dropEffect);
       }
@@ -3534,8 +3533,7 @@ EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
       // the initial dataTransfer is the one from the dragstart event that
       // was set on the dragSession when the drag began.
       nsCOMPtr<nsIDOMDataTransfer> dataTransfer;
-      nsCOMPtr<nsIDOMDataTransfer> initialDataTransfer;
-      dragSession->GetDataTransfer(getter_AddRefs(initialDataTransfer));
+      RefPtr<DataTransfer> initialDataTransfer = dragSession->GetDataTransfer();
 
       WidgetDragEvent *dragEvent = aEvent->AsDragEvent();
 
@@ -4781,8 +4779,7 @@ EventStateManager::UpdateDragDataTransfer(WidgetDragEvent* dragEvent)
   if (dragSession) {
     // the initial dataTransfer is the one from the dragstart event that
     // was set on the dragSession when the drag began.
-    nsCOMPtr<nsIDOMDataTransfer> initialDataTransfer;
-    dragSession->GetDataTransfer(getter_AddRefs(initialDataTransfer));
+    RefPtr<DataTransfer> initialDataTransfer = dragSession->GetDataTransfer();
     if (initialDataTransfer) {
       // retrieve the current moz cursor setting and save it.
       nsAutoString mozCursor;
diff --git a/dom/interfaces/events/nsIDOMDataTransfer.idl b/dom/interfaces/events/nsIDOMDataTransfer.idl
index 82ac58d5734e..3eafea7284df 100644
--- a/dom/interfaces/events/nsIDOMDataTransfer.idl
+++ b/dom/interfaces/events/nsIDOMDataTransfer.idl
@@ -68,20 +68,6 @@ interface nsIDOMDataTransfer : nsISupports
    */
   void addElement(in nsIDOMElement element);
 
-  /**
-   * Sets the drag cursor state. Primarily used to control the cursor during
-   * tab drags, but could be expanded to other uses. XXX Currently implemented
-   * on Win32 only.
-   *
-   * Possible values:
-   *  auto - use default system behavior.
-   *  default - set the cursor to an arrow during the drag operation.
-   *
-   * Values other than 'default' are indentical to setting mozCursor to
-   * 'auto'.
-   */
-  attribute DOMString mozCursor;
-
   /**
    * Will be true when the user has cancelled the drag (typically by pressing
    * Escape) and when the drag has been cancelled unexpectedly.  This will be
diff --git a/dom/ipc/ContentParent.cpp b/dom/ipc/ContentParent.cpp
index 4b432f892af1..e361eb7b7a54 100644
--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -4512,9 +4512,7 @@ ContentParent::MaybeInvokeDragSession(TabParent* aParent)
     dragService->GetCurrentSession(getter_AddRefs(session));
     if (session) {
       nsTArray<IPCDataTransfer> dataTransfers;
-      nsCOMPtr<nsIDOMDataTransfer> domTransfer;
-      session->GetDataTransfer(getter_AddRefs(domTransfer));
-      nsCOMPtr<DataTransfer> transfer = do_QueryInterface(domTransfer);
+      RefPtr<DataTransfer> transfer = session->GetDataTransfer();
       if (!transfer) {
         // Pass eDrop to get DataTransfer with external
         // drag formats cached.
@@ -4548,8 +4546,7 @@ ContentParent::RecvUpdateDropEffect(const uint32_t& aDragAction,
   nsCOMPtr<nsIDragSession> dragSession = nsContentUtils::GetDragSession();
   if (dragSession) {
     dragSession->SetDragAction(aDragAction);
-    nsCOMPtr<nsIDOMDataTransfer> dt;
-    dragSession->GetDataTransfer(getter_AddRefs(dt));
+    RefPtr<DataTransfer> dt = dragSession->GetDataTransfer();
     if (dt) {
       dt->SetDropEffectInt(aDropEffect);
     }
diff --git a/dom/ipc/TabChild.cpp b/dom/ipc/TabChild.cpp
index 50725636d280..9d5e81bea3d1 100644
--- a/dom/ipc/TabChild.cpp
+++ b/dom/ipc/TabChild.cpp
@@ -19,6 +19,7 @@
 #include "mozilla/BrowserElementParent.h"
 #include "mozilla/ClearOnShutdown.h"
 #include "mozilla/EventListenerManager.h"
+#include "mozilla/dom/DataTransfer.h"
 #include "mozilla/dom/indexedDB/PIndexedDBPermissionRequestChild.h"
 #include "mozilla/dom/PaymentRequestChild.h"
 #include "mozilla/dom/TelemetryScrollProbe.h"
@@ -1955,8 +1956,7 @@ TabChild::RecvRealDragEvent(const WidgetDragEvent& aEvent,
   if (dragSession) {
     dragSession->SetDragAction(aDragAction);
     dragSession->SetTriggeringPrincipalURISpec(aPrincipalURISpec);
-    nsCOMPtr<nsIDOMDataTransfer> initialDataTransfer;
-    dragSession->GetDataTransfer(getter_AddRefs(initialDataTransfer));
+    RefPtr<DataTransfer> initialDataTransfer = dragSession->GetDataTransfer();
     if (initialDataTransfer) {
       initialDataTransfer->SetDropEffectInt(aDropEffect);
     }
diff --git a/dom/ipc/TabParent.cpp b/dom/ipc/TabParent.cpp
index d4e3d11ea2e9..65b2c6ab33fd 100644
--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -1179,8 +1179,7 @@ TabParent::QueryDropLinksForVerification()
     return false;
   }
 
-  nsCOMPtr<nsIDOMDataTransfer> initialDataTransfer;
-  dragSession->GetDataTransfer(getter_AddRefs(initialDataTransfer));
+  RefPtr<DataTransfer> initialDataTransfer = dragSession->GetDataTransfer();
   if (!initialDataTransfer) {
     NS_WARNING("No initialDataTransfer to query links for verification");
     return false;
diff --git a/widget/cocoa/nsChildView.mm b/widget/cocoa/nsChildView.mm
index fd55ccba3e50..9d8a117dcc85 100644
--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -17,6 +17,7 @@
 #include "mozilla/MouseEvents.h"
 #include "mozilla/TextEvents.h"
 #include "mozilla/TouchEvents.h"
+#include "mozilla/dom/DataTransfer.h"
 #include "mozilla/dom/SimpleGestureEventBinding.h"
 
 #include "nsArrayUtils.h"
@@ -6260,10 +6261,10 @@ GetIntegerDeltaForEvent(NSEvent* aEvent)
     // value for NSDragOperationGeneric that is passed by other applications.
     // All that said, NSDragOperationNone is still reliable.
     if (aOperation == NSDragOperationNone) {
-      nsCOMPtr<nsIDOMDataTransfer> dataTransfer;
-      dragService->GetDataTransfer(getter_AddRefs(dataTransfer));
-      if (dataTransfer)
+      RefPtr<dom::DataTransfer> dataTransfer = dragService->GetDataTransfer();
+      if (dataTransfer) {
         dataTransfer->SetDropEffectInt(nsIDragService::DRAGDROP_ACTION_NONE);
+      }
     }
 
     mDragService->EndDragSession(
diff --git a/widget/nsBaseDragService.cpp b/widget/nsBaseDragService.cpp
index 727b975895ef..e730c407f897 100644
--- a/widget/nsBaseDragService.cpp
+++ b/widget/nsBaseDragService.cpp
@@ -23,7 +23,6 @@
 #include "nsISelection.h"
 #include "nsISelectionPrivate.h"
 #include "nsPresContext.h"
-#include "nsIDOMDataTransfer.h"
 #include "nsIImageLoadingContent.h"
 #include "imgIContainer.h"
 #include "imgIRequest.h"
@@ -35,6 +34,7 @@
 #include "mozilla/MouseEvents.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/dom/DataTransferItemList.h"
+#include "mozilla/dom/DataTransfer.h"
 #include "mozilla/gfx/2D.h"
 #include "mozilla/Unused.h"
 #include "nsFrameLoader.h"
@@ -201,7 +201,7 @@ nsBaseDragService::IsDataFlavorSupported(const char *aDataFlavor,
 }
 
 NS_IMETHODIMP
-nsBaseDragService::GetDataTransfer(nsIDOMDataTransfer** aDataTransfer)
+nsBaseDragService::GetDataTransferXPCOM(nsIDOMDataTransfer** aDataTransfer)
 {
   *aDataTransfer = mDataTransfer;
   NS_IF_ADDREF(*aDataTransfer);
@@ -209,10 +209,23 @@ nsBaseDragService::GetDataTransfer(nsIDOMDataTransfer** aDataTransfer)
 }
 
 NS_IMETHODIMP
-nsBaseDragService::SetDataTransfer(nsIDOMDataTransfer* aDataTransfer)
+nsBaseDragService::SetDataTransferXPCOM(nsIDOMDataTransfer* aDataTransfer)
+{
+  // Cast is safe because nsIDOMDataTransfer is builtinclass.
+  mDataTransfer = static_cast<DataTransfer*>(aDataTransfer);
+  return NS_OK;
+}
+
+DataTransfer*
+nsBaseDragService::GetDataTransfer()
+{
+  return mDataTransfer;
+}
+
+void
+nsBaseDragService::SetDataTransfer(DataTransfer* aDataTransfer)
 {
   mDataTransfer = aDataTransfer;
-  return NS_OK;
 }
 
 //-------------------------------------------------------------------------
@@ -271,7 +284,8 @@ nsBaseDragService::InvokeDragSessionWithImage(nsIDOMNode* aDOMNode,
   NS_ENSURE_TRUE(aDataTransfer, NS_ERROR_NULL_POINTER);
   NS_ENSURE_TRUE(mSuppressLevel == 0, NS_ERROR_FAILURE);
 
-  mDataTransfer = aDataTransfer;
+  // Cast is safe because nsIDOMDataTransfer is builtinclass.
+  mDataTransfer = static_cast<DataTransfer*>(aDataTransfer);
   mSelection = nullptr;
   mHasImage = true;
   mDragPopup = nullptr;
@@ -308,7 +322,8 @@ nsBaseDragService::InvokeDragSessionWithSelection(nsISelection* aSelection,
   NS_ENSURE_TRUE(aDragEvent, NS_ERROR_NULL_POINTER);
   NS_ENSURE_TRUE(mSuppressLevel == 0, NS_ERROR_FAILURE);
 
-  mDataTransfer = aDataTransfer;
+  // Cast is safe because nsIDOMDataTransfer is builtinclass.
+  mDataTransfer = static_cast<DataTransfer*>(aDataTransfer);
   mSelection = aSelection;
   mHasImage = true;
   mDragPopup = nullptr;
diff --git a/widget/nsBaseDragService.h b/widget/nsBaseDragService.h
index 3ba847c9be2b..4a578c2ee7d2 100644
--- a/widget/nsBaseDragService.h
+++ b/widget/nsBaseDragService.h
@@ -10,7 +10,6 @@
 #include "nsIDragSession.h"
 #include "nsITransferable.h"
 #include "nsIDOMDocument.h"
-#include "nsIDOMDataTransfer.h"
 #include "nsCOMPtr.h"
 #include "nsRect.h"
 #include "nsPoint.h"
@@ -33,6 +32,10 @@ namespace mozilla {
 namespace gfx {
 class SourceSurface;
 } // namespace gfx
+
+namespace dom {
+class DataTransfer;
+} // namespace dom
 } // namespace mozilla
 
 /**
@@ -165,7 +168,7 @@ protected:
                                                   //  if it came from outside the app.
   nsContentPolicyType mContentPolicyType;         // the contentpolicy type passed to the channel
                                                   // when initiating the drag session
-  nsCOMPtr<nsIDOMDataTransfer> mDataTransfer;
+  RefPtr<mozilla::dom::DataTransfer> mDataTransfer;
 
   // used to determine the image to appear on the cursor while dragging
   nsCOMPtr<nsIDOMNode> mImage;
diff --git a/widget/nsIDragSession.idl b/widget/nsIDragSession.idl
index 316736fa4b2e..47c57093568c 100644
--- a/widget/nsIDragSession.idl
+++ b/widget/nsIDragSession.idl
@@ -10,16 +10,22 @@
 
 %{ C++
 #include "nsSize.h"
+
+namespace mozilla {
+namespace dom {
+class DataTransfer;
+} // namespace dom
+} // namespace mozilla
 %}
 
 native nsSize (nsSize);
-
+[ptr] native DataTransferPtr(mozilla::dom::DataTransfer);
 
 interface nsIDOMDocument;
 interface nsIDOMNode;
 interface nsIDOMDataTransfer;
 
-[scriptable, uuid(25bce737-73f0-43c7-bc20-c71044a73c5a)]
+[scriptable, builtinclass, uuid(25bce737-73f0-43c7-bc20-c71044a73c5a)]
 interface nsIDragSession : nsISupports
 {
   /**
@@ -72,7 +78,10 @@ interface nsIDragSession : nsISupports
   /**
    * The data transfer object for the current drag.
    */
+  [binaryname(DataTransferXPCOM)]
   attribute nsIDOMDataTransfer dataTransfer;
+  [notxpcom, nostdcall] DataTransferPtr getDataTransfer();
+  [notxpcom, nostdcall] void setDataTransfer(in DataTransferPtr aDataTransfer);
 
   /**
     * Get data from a Drag&Drop. Can be called while the drag is in process
diff --git a/widget/windows/nsNativeDragSource.cpp b/widget/windows/nsNativeDragSource.cpp
index 857907076703..380a3077cd91 100644
--- a/widget/windows/nsNativeDragSource.cpp
+++ b/widget/windows/nsNativeDragSource.cpp
@@ -15,12 +15,12 @@
 /*
  * class nsNativeDragSource
  */
-nsNativeDragSource::nsNativeDragSource(nsIDOMDataTransfer* aDataTransfer) :
+nsNativeDragSource::nsNativeDragSource(mozilla::dom::DataTransfer* aDataTransfer) :
   m_cRef(0),
   m_hCursor(nullptr),
   mUserCancelled(false)
 {
-  mDataTransfer = do_QueryInterface(aDataTransfer);
+  mDataTransfer = aDataTransfer;
 }
 
 nsNativeDragSource::~nsNativeDragSource()
diff --git a/widget/windows/nsNativeDragSource.h b/widget/windows/nsNativeDragSource.h
index 9172291fb129..9784c4e09306 100644
--- a/widget/windows/nsNativeDragSource.h
+++ b/widget/windows/nsNativeDragSource.h
@@ -7,10 +7,16 @@
 
 #include "nscore.h"
 #include "nsIDOMDataTransfer.h"
-#include "nsCOMPtr.h"
 #include <ole2.h>
 #include <oleidl.h>
 #include "mozilla/Attributes.h"
+#inclide "mozilla/RefPtr.h"
+
+namespace mozilla {
+namespace dom {
+class DataTransfer;
+} // namespace dom
+} // namespace mozilla
 
 //class nsIDragSource;
 
@@ -24,7 +30,7 @@ public:
 
   // construct an nsNativeDragSource referencing adapter
   // nsNativeDragSource(nsIDragSource * adapter);
-  explicit nsNativeDragSource(nsIDOMDataTransfer* aDataTransfer);
+  explicit nsNativeDragSource(mozilla::dom::DataTransfer* aDataTransfer);
   ~nsNativeDragSource();
 
   // IUnknown methods - see iunknown.h for documentation
@@ -53,7 +59,7 @@ protected:
   ULONG m_cRef;
 
   // Data object, hold information about cursor state
-  nsCOMPtr<nsIDOMDataTransfer> mDataTransfer;
+  RefPtr<mozilla::dom::DataTransfer> mDataTransfer;
 
   // Custom drag cursor
   HCURSOR m_hCursor;