diff --git a/content/treestyletab/treestyletab.js b/content/treestyletab/treestyletab.js
index 9f247a2e..a364d61a 100644
--- a/content/treestyletab/treestyletab.js
+++ b/content/treestyletab/treestyletab.js
@@ -648,15 +648,19 @@ catch(e) {
 	onTabDNDObserverDragStart : function TSTService_onTabDNDObserverDragStart(aEvent) 
 	{
 		var tabbar = aEvent.currentTarget;
-		if (tabbar.treeStyleTab.tabbarDNDObserver.canDragTabbar(aEvent))
+		if (tabbar.treeStyleTab.tabbarDNDObserver.canDragTabbar(aEvent)) {
 			tabbar.treeStyleTab.onTabbarDragStart(aEvent, tabbar.tabbrowser);
+			aEvent.preventDefault();
+		}
 	},
  
 	onTabDNDObserverDragOver : function TSTService_onTabDNDObserverDragOver(aEvent) 
 	{
 		var tabbar = aEvent.currentTarget;
-		if (tabbar.treeStyleTab.processTabDragOverEvent(aEvent, tabbar.tabbrowser))
+		if (tabbar.treeStyleTab.processTabDragOverEvent(aEvent, tabbar.tabbrowser)) {
 			aEvent.stopPropagation();
+			aEvent.preventDefault();
+		}
 	},
  
 	onTabDNDObserverDragLeave : function TSTService_onTabDNDObserverDragLeave(aEvent) 
@@ -686,6 +690,8 @@ catch(e) {
 				return;
 			}
 		}
+		if (!draggedTab)
+			return;
 
 		if (sv.performDrop(dropActionInfo, draggedTab)) {
 			aEvent.stopPropagation();