diff --git a/js/lib/utils.js b/js/lib/utils.js
--- a/js/lib/utils.js
+++ b/js/lib/utils.js
@@ -1247,16 +1247,23 @@ function isinstance(inst, base)
      * 254067 which makes instanceof fail if the two sides are 'from'
      * different windows (something we don't care about).
      */
     return (inst && base &&
             ((inst instanceof base) ||
              (inst.constructor && (inst.constructor.name == base.name))));
 }
 
+function isDefaultPrevented(ev)
+{
+    if ("defaultPrevented" in ev)
+        return ev.defaultPrevented;
+    return ev.getPreventDefault();
+}
+
 function scaleNumberBy1024(number)
 {
     var scale = 0;
     while ((number >= 1000) && (scale < 6))
     {
         scale++;
         number /= 1024;
     }
diff --git a/xul/content/static.js b/xul/content/static.js
--- a/xul/content/static.js
+++ b/xul/content/static.js
@@ -3826,17 +3826,17 @@ function ul_getcellprops(index, column, 
     });
 }
 
 var contentDropObserver = new Object();
 
 contentDropObserver.onDragOver =
 function cdnd_dover(aEvent, aFlavour, aDragSession)
 {
-    if (aEvent.getPreventDefault())
+    if (isDefaultPrevented(aEvent))
         return;
 
     if (aDragSession.sourceDocument == aEvent.view.document)
     {
         aDragSession.canDrop = false;
         return;
     }
 }
@@ -3890,33 +3890,33 @@ function cdnd_gsf()
  * significantly impact the performance of the drag (getting the data can be
  * very slow).
  */
 var tabsDropObserver = new Object();
 
 tabsDropObserver.canDrop =
 function tabdnd_candrop(aEvent, aDragSession)
 {
-    if (aEvent.getPreventDefault())
+    if (isDefaultPrevented(aEvent))
         return false;
 
     // See comment above |var tabsDropObserver|.
     var flavourSet = this.getSupportedFlavours();
     for (var flavour in flavourSet.flavourTable)
     {
         if (aDragSession.isDataFlavorSupported(flavour))
             return true;
     }
     return false;
 }
 
 tabsDropObserver.onDragOver =
 function tabdnd_dover(aEvent, aFlavour, aDragSession)
 {
-    if (aEvent.getPreventDefault())
+    if (isDefaultPrevented(aEvent))
         return;
 
     // See comment above |var tabsDropObserver|.
     if (aDragSession.sourceNode == aEvent.target)
         aDragSession.canDrop = true;
 
     // If we're not accepting the drag, don't show the marker either.
     if (!aDragSession.canDrop)
@@ -3948,17 +3948,17 @@ function tabdnd_dover(aEvent, aFlavour, 
     // Reposition the drop marker and show it. In that order.
     client.tabDragMarker.style.MozMarginStart = newPosition + "px";
     client.tabDragBar.collapsed = false;
 }
 
 tabsDropObserver.onDragExit =
 function tabdnd_dexit(aEvent, aDragSession)
 {
-    if (aEvent.getPreventDefault())
+    if (isDefaultPrevented(aEvent))
         return;
 
     /* We've either stopped being part of a drag operation, or the dragging is
      * somewhere away from us.
      */
     client.tabDragBar.collapsed = true;
 }