diff --git a/toolkit/content/widgets/tabbrowser.xml b/toolkit/content/widgets/tabbrowser.xml
index 410a9646fa3..b04bcde8b95 100644
--- a/toolkit/content/widgets/tabbrowser.xml
+++ b/toolkit/content/widgets/tabbrowser.xml
@@ -1483,7 +1483,8 @@
         <parameter name="aDragSession"/>
         <body>
           <![CDATA[
-            if (aDragSession.canDrop && aDragSession.sourceNode.parentNode == this.mTabContainer) {
+            if (aDragSession.canDrop && aDragSession.sourceNode &&
+                aDragSession.sourceNode.parentNode == this.mTabContainer) {
               var newIndex = this.getNewIndex(aEvent);
 
               var ib = this.mTabDropIndicatorBar;
