diff --git a/content/treestyletab/preloadImages.js b/content/treestyletab/preloadImages.js
index 9a202539..8e58dbd0 100644
--- a/content/treestyletab/preloadImages.js
+++ b/content/treestyletab/preloadImages.js
@@ -30,7 +30,7 @@ TreeStyleTabService._preLoadImages = {
 		'chrome://treestyletab/skin/metal/shadow-active-l.png',
 		'chrome://treestyletab/skin/metal/shadow-inactive-l.png'
 	].concat(
-		'MozBorderImage' in document.documentElement.style ?
+		TreeStyleTabService.Comparator.compare(TreeStyleTabService.XULAppInfo.version, '3.5') >= 0 ?
 			[
 				'chrome://treestyletab/skin/metal/tab-active-middle.png',
 				'chrome://treestyletab/skin/metal/tab-active-middle-selected.png',
@@ -47,7 +47,7 @@ TreeStyleTabService._preLoadImages = {
 		'chrome://treestyletab/skin/metal/shadow-active-r.png',
 		'chrome://treestyletab/skin/metal/shadow-inactive-r.png'
 	].concat(
-		'MozBorderImage' in document.documentElement.style ?
+		TreeStyleTabService.Comparator.compare(TreeStyleTabService.XULAppInfo.version, '3.5') >= 0 ?
 			[
 				'chrome://treestyletab/skin/metal/tab-active-middle.png',
 				'chrome://treestyletab/skin/metal/tab-active-middle-selected.png',
diff --git a/content/treestyletab/treestyletab.js b/content/treestyletab/treestyletab.js
index 9226a25e..781113d6 100644
--- a/content/treestyletab/treestyletab.js
+++ b/content/treestyletab/treestyletab.js
@@ -9,8 +9,11 @@ var TreeStyleTabService = {
 	
 	get currentTabbarPosition() /* PUBLIC API */ 
 	{
-		return this.browser.getAttribute(this.kTABBAR_POSITION) ||
-				this.utils.currentTabbarPosition;
+		return (
+			// Don't touch to the <tabbrowser/> element before it is initialized by XBL constructor.
+			(this.preInitialized && this.browser.getAttribute(this.kTABBAR_POSITION)) ||
+			this.utils.currentTabbarPosition
+		);
 	},
 	set currentTabbarPosition(aValue)
 	{