diff --git a/xul/content/nsClipboard.js b/xul/content/nsClipboard.js
--- a/xul/content/nsClipboard.js
+++ b/xul/content/nsClipboard.js
@@ -36,29 +36,29 @@
   read: function (aFlavourList, aClipboard, aAnyFlag)
     {
       this.currentClipboard = aClipboard;
       var data = nsTransferable.get(aFlavourList, this.getClipboardTransferable, aAnyFlag);
       return data.first.first;  // only support one item
     },
 
   /**
-   * nsISupportsArray getClipboardTransferable (Object aFlavourList) ;
+   * nsIArray getClipboardTransferable (Object aFlavourList) ;
    *
-   * returns a nsISupportsArray of the item on the clipboard
+   * returns a nsIArray of the item on the clipboard
    *
    * @param Object aFlavourList
    *        formatted list of desired flavours.
    **/
   getClipboardTransferable: function (aFlavourList)
     {
-      const supportsContractID = "@mozilla.org/supports-array;1";
-      const supportsIID = Components.interfaces.nsISupportsArray;
-      var supportsArray = Components.classes[supportsContractID].createInstance(supportsIID);
+      const arrayContractID = "@mozilla.org/array;1";
+      const mutableIID = Components.interfaces.nsIMutableArray;
+      let mutableArray = Components.classes[arrayContractID].createInstance(mutableIID);
       var trans = nsTransferable.createTransferable();
       for (var flavour in aFlavourList)
         trans.addDataFlavor(flavour);
       nsClipboard.mClipboard.getData(trans, nsClipboard.currentClipboard)
-      supportsArray.AppendElement(trans);
-      return supportsArray;
+      mutableArray.appendElement(trans, /* weak */ false);
+      return mutableArray;
     }
 };
 
diff --git a/xul/content/nsDragAndDrop.js b/xul/content/nsDragAndDrop.js
--- a/xul/content/nsDragAndDrop.js
+++ b/xul/content/nsDragAndDrop.js
@@ -65,40 +65,40 @@
    *                                   Function aRetrievalFunc, Boolean aAnyFlag) ;
    *
    * Retrieves data from the transferable provided in aRetrievalFunc, formatted
    * for more convenient access.
    *
    * @param FlavourSet aFlavourSet
    *        a FlavourSet object that contains a list of supported flavours.
    * @param Function aRetrievalFunc
-   *        a reference to a function that returns a nsISupportsArray of nsITransferables
+   *        a reference to a function that returns a nsIArray of nsITransferables
    *        for each item from the specified source (clipboard/drag&drop etc)
    * @param Boolean aAnyFlag
    *        a flag specifying whether or not a specific flavour is requested. If false,
    *        data of the type of the first flavour in the flavourlist parameter is returned,
    *        otherwise the best flavour supported will be returned.
    **/
   get: function (aFlavourSet, aRetrievalFunc, aAnyFlag)
     {
       if (!aRetrievalFunc)
         throw "No data retrieval handler provided!";
 
-      var supportsArray = aRetrievalFunc(aFlavourSet);
+      let array = aRetrievalFunc(aFlavourSet);
       var dataArray = [];
-      var count = supportsArray.Count();
+      let count = array.length;
 
       // Iterate over the number of items returned from aRetrievalFunc. For
       // clipboard operations, this is 1, for drag and drop (where multiple
       // items may have been dragged) this could be >1.
       for (var i = 0; i < count; i++)
         {
-          var trans = supportsArray.GetElementAt(i);
-          if (!trans) continue;
-          trans = trans.QueryInterface(Components.interfaces.nsITransferable);
+          let trans = array.queryElementAt(i, Components.interfaces.nsITransferable);
+          if (!trans)
+            continue;
 
           var data = { };
           var length = { };
 
           var currData = null;
           if (aAnyFlag)
             {
               var flavour = { };