--- a/dom/base/Selection.h
+++ b/dom/base/Selection.h
@@ -51,29 +51,30 @@ struct RangeData
 // Note, the ownership of mozilla::dom::Selection depends on which way the
 // object is created. When nsFrameSelection has created Selection,
 // addreffing/releasing the Selection object is aggregated to nsFrameSelection.
 // Otherwise normal addref/release is used.  This ensures that nsFrameSelection
 // is never deleted before its Selections.
 namespace mozilla {
 namespace dom {
 
-class Selection final : public nsISelectionPrivate,
+class Selection final : public nsISelection,
                         public nsWrapperCache,
+                        public nsISelectionPrivate,
                         public nsSupportsWeakReference
 {
 protected:
   virtual ~Selection();
 
 public:
   Selection();
   explicit Selection(nsFrameSelection *aList);
 
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
-  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_AMBIGUOUS(Selection, nsISelectionPrivate)
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_AMBIGUOUS(Selection, nsISelection)
   NS_DECL_NSISELECTION
   NS_DECL_NSISELECTIONPRIVATE
 
   // match this up with EndbatchChanges. will stop ui updates while multiple
   // selection methods are called
   void StartBatchChanges();
 
   // match this up with StartBatchChanges

--- a/dom/base/nsISelectionPrivate.idl
+++ b/dom/base/nsISelectionPrivate.idl
@@ -23,17 +23,17 @@ struct ScrollAxis;
 
 [ptr] native nsIFrame(nsIFrame);
 [ptr] native RangeArray(nsTArray<nsRange*>);
 [ref] native nsPointRef(nsPoint);
 native nsDirection(nsDirection);
 native ScrollAxis(nsIPresShell::ScrollAxis);
 
 [scriptable, builtinclass, uuid(0c9f4f74-ee7e-4fe9-be6b-0ba856368178)]
-interface nsISelectionPrivate : nsISelection
+interface nsISelectionPrivate : nsISupports
  {
     const short ENDOFPRECEDINGLINE=0;
     const short STARTOFNEXTLINE=1;
 
     attribute boolean interlinePosition;
     [noscript] attribute nsIContent ancestorLimiter;
 
     DOMString  toStringWithFormat(in string formatType, in unsigned long flags, in int32_t wrapColumn);