diff --git a/content/treestyletab/treestyletab.js b/content/treestyletab/treestyletab.js
index 2b476529..5e18ec47 100644
--- a/content/treestyletab/treestyletab.js
+++ b/content/treestyletab/treestyletab.js
@@ -1323,19 +1323,7 @@ catch(e) {
 		event.initEvent('TreeStyleTabSubtreeClosing', true, true);
 		event.parent = aParentTab;
 		event.tabs = aClosedTabs;
-		if (!event.getPreventDefault) {
-			// getPreventDefault is available on any event on Gecko 1.9.2 or later.
-			// on Gecko 1.9.1 or before, UIEvents only have the method...
-			event.__original__preventDefault = event.preventDefault;
-			event.__canceled = false;
-			event.preventDefault = function() {
-				this.__original__preventDefault();
-				this.__canceled = true;
-			};
-			event.getPreventDefault = function() {
-				return this.__canceled;
-			};
-		}
+		this.ensureEventCancelable(event);
 		this.getTabBrowserFromChild(aParentTab).dispatchEvent(event);
 		return !event.getPreventDefault();
 	},
diff --git a/modules/utils.js b/modules/utils.js
index c39536bf..addabcb3 100644
--- a/modules/utils.js
+++ b/modules/utils.js
@@ -280,7 +280,7 @@ var TreeStyleTabUtils = {
 		return boxObject.getBoxObjectFor(aNode);
 	},
  
-	evalInSandbox : function TSTUtils_evalInSandbox(aCode, aOwner)
+	evalInSandbox : function TSTUtils_evalInSandbox(aCode, aOwner) 
 	{
 		try {
 			var sandbox = new Components.utils.Sandbox(aOwner || 'about:blank');
@@ -379,6 +379,22 @@ var TreeStyleTabUtils = {
  
 // event 
 	
+	ensureEventCancelable : function(aEvent) 
+	{
+		if (aEvent.getPreventDefault) return;
+		// getPreventDefault is available on any event on Gecko 1.9.2 or later.
+		// on Gecko 1.9.1 or before, UIEvents only have the method...
+		aEvent.__original__preventDefault = aEvent.preventDefault;
+		aEvent.__canceled = false;
+		aEvent.preventDefault = function() {
+			this.__original__preventDefault();
+			this.__canceled = true;
+		};
+		aEvent.getPreventDefault = function() {
+			return this.__canceled;
+		};
+	},
+ 
 	isNewTabAction : function TSTUtils_isNewTabAction(aEvent) 
 	{
 		return aEvent.button == 1 || (aEvent.button == 0 && this.isAccelKeyPressed(aEvent));
@@ -1494,7 +1510,7 @@ var TreeStyleTabUtils = {
 	_tabbarPositionHistory : [], 
 	_tabbarPositionHistoryIndex : -1,
  
-	kMAX_TABBAR_POSITION_HISTORY : 999,
+	kMAX_TABBAR_POSITION_HISTORY : 999, 
   
 /* Pref Listener */