diff --git a/browser/components/sessionstore/content/aboutSessionRestore.js b/browser/components/sessionstore/content/aboutSessionRestore.js
index ae5535374920..bbbe87b08d17 100644
--- a/browser/components/sessionstore/content/aboutSessionRestore.js
+++ b/browser/components/sessionstore/content/aboutSessionRestore.js
@@ -289,6 +289,7 @@ var treeView = {
   isSeparator: function(idx)         { return false; },
   isSorted: function()               { return false; },
   isEditable: function(idx, column)  { return false; },
+  canDrop: function(idx, orientation, dt) { return false; },
   getLevel: function(idx)            { return this.isContainer(idx) ? 0 : 1; },
 
   getParentIndex: function(idx) {