diff --git a/content/treestyletab/treestyletab.js b/content/treestyletab/treestyletab.js
index 6f87d23c..436d7121 100644
--- a/content/treestyletab/treestyletab.js
+++ b/content/treestyletab/treestyletab.js
@@ -142,77 +142,97 @@ var TreeStyleTabService = {
 	},
 
 	get SessionStore() {
-		delete this.SessionStore;
-		return (this.SessionStore = Components
+		if (!this._SessionStore) {
+			this._SessionStore = Components
 					.classes['@mozilla.org/browser/sessionstore;1']
-					.getService(Components.interfaces.nsISessionStore));
+					.getService(Components.interfaces.nsISessionStore);
+		}
+		return this._SessionStore;
 	},
+	_SessionStore : null,
 
 	get ObserverService() {
-		delete this.ObserverService;
-		return (this.ObserverService = Components
+		if (!this._ObserverService) {
+			this._ObserverService = Components
 					.classes['@mozilla.org/observer-service;1']
-					.getService(Components.interfaces.nsIObserverService));
+					.getService(Components.interfaces.nsIObserverService);
+		}
+		return this._ObserverService;
 	},
+	_ObserverService : null,
 
 	get IOService() {
-		delete this.IOService;
-		return (this.IOService = Components
+		if (!this._IOService) {
+			this._IOService = Components
 					.classes['@mozilla.org/network/io-service;1']
-					.getService(Components.interfaces.nsIIOService));
+					.getService(Components.interfaces.nsIIOService);
+		}
+		return this._IOService;
 	},
+	_IOService : null,
 
 	get WindowMediator() {
-		delete this.WindowMediator;
-		return (this.WindowMediator = Components
+		if (!this._WindowMediator) {
+			this._WindowMediator = Components
 					.classes['@mozilla.org/appshell/window-mediator;1']
-					.getService(Components.interfaces.nsIWindowMediator));
+					.getService(Components.interfaces.nsIWindowMediator);
+		}
+		return this._WindowMediator;
 	},
+	_WindowMediator : null,
 
 	get EffectiveTLD()
 	{
-		delete this.EffectiveTLD;
-		return (this.EffectiveTLD = (
-					'nsIEffectiveTLDService' in Components.interfaces ?
-						Components
-							.classes['@mozilla.org/network/effective-tld-service;1']
-							.getService(Components.interfaces.nsIEffectiveTLDService) :
-						null
-				));
+		if (!('_EffectiveTLD' in this)) {
+			this._EffectiveTLD = 'nsIEffectiveTLDService' in Components.interfaces ?
+				Components
+					.classes['@mozilla.org/network/effective-tld-service;1']
+					.getService(Components.interfaces.nsIEffectiveTLDService) :
+				null ;
+		}
+		return this._EffectiveTLD;
 	},
+//	_EffectiveTLD : null,
 
 	get PromptService()
 	{
-		delete this.PromptService;
-		return (this.PromptService = Components
+		if (!this._PromptService) {
+			this._PromptService = Components
 					.classes['@mozilla.org/embedcomp/prompt-service;1']
-					.getService(Components.interfaces.nsIPromptService));
+					.getService(Components.interfaces.nsIPromptService);
+		}
+		return this._PromptService;
 	},
+	_PromptService : null,
 
 	get XULAppInfo() {
-		delete this.XULAppInfo;
-		return (this.XULAppInfo = Components
+		if (!this._XULAppInfo) {
+			this._XULAppInfo = Components
 					.classes['@mozilla.org/xre/app-info;1']
-					.getService(Components.interfaces.nsIXULAppInfo));
+					.getService(Components.interfaces.nsIXULAppInfo);
+		}
+		return this._XULAppInfo;
 	},
+	_XULAppInfo : null,
 	get Comparator() {
-		delete this.Comparator;
-		return (this.Comparator = Components
+		if (!this._Comparator) {
+			this._Comparator = Components
 					.classes['@mozilla.org/xpcom/version-comparator;1']
-					.getService(Components.interfaces.nsIVersionComparator));
+					.getService(Components.interfaces.nsIVersionComparator);
+		}
+		return this._Comparator;
 	},
+	_Comparator : null,
 
 	get treeBundle() {
-		delete this.treeBundle;
-		return (this.treeBundle = window['piro.sakura.ne.jp']
+		return window['piro.sakura.ne.jp']
 				.stringBundle
-				.get('chrome://treestyletab/locale/treestyletab.properties'));
+				.get('chrome://treestyletab/locale/treestyletab.properties');
 	},
 	get tabbrowserBundle() {
-		delete this.tabbrowserBundle;
-		return (this.tabbrowserBundle = window['piro.sakura.ne.jp']
+		return window['piro.sakura.ne.jp']
 				.stringBundle
-				.get('chrome://browser/locale/tabbrowser.properties'));
+				.get('chrome://browser/locale/tabbrowser.properties');
 	},
 	
 /* API */ 
diff --git a/content/treestyletab/treestyletabbrowser.js b/content/treestyletab/treestyletabbrowser.js
index 841f0289..f0e971de 100644
--- a/content/treestyletab/treestyletabbrowser.js
+++ b/content/treestyletab/treestyletabbrowser.js
@@ -54,17 +54,22 @@ TreeStyleTabBrowser.prototype = {
  
 	get container() 
 	{
-		delete this.container;
-		return (this.container = document.getElementById('appcontent'));
+		if (!this._container) {
+			this._container = document.getElementById('appcontent');
+		}
+		return this._container;
 	},
+	_container : null,
  
 	get scrollBox() 
 	{
-		delete this.scrollBox;
-		var scrollBox = document.getAnonymousElementByAttribute(this.mTabBrowser.mTabContainer, 'class', 'tabs-frame') || // Tab Mix Plus
+		if (!this._scrollBox) {
+			this._scrollBox = document.getAnonymousElementByAttribute(this.mTabBrowser.mTabContainer, 'class', 'tabs-frame') || // Tab Mix Plus
 						this.mTabBrowser.mTabContainer.mTabstrip;
-		return (this.scrollBox = scrollBox);
+		}
+		return this._scrollBox;
 	},
+	_scrollBox : null,
 	get scrollBoxObject()
 	{
 		return (this.scrollBox.scrollBoxObject || this.scrollBox.boxObject)
@@ -1024,9 +1029,9 @@ TreeStyleTabBrowser.prototype = {
 		b.mPanelContainer.removeEventListener('dragdrop', this, false);
 
 		this.tabbarDNDObserver.destroy();
-		delete this.tabbarDNDObserver;
+		delete this._tabbarDNDObserver;
 		this.panelDNDObserver.destroy();
-		delete this.panelDNDObserver;
+		delete this._panelDNDObserver;
 
 		this.scrollBox.removeEventListener('overflow', this, true);
 		this.scrollBox.removeEventListener('underflow', this, true);
@@ -2345,14 +2350,12 @@ TreeStyleTabBrowser.prototype = {
 	
 	get tabbarDNDObserver() 
 	{
-		delete this.tabbarDNDObserver;
-		return (this.tabbarDNDObserver = new TreeStyleTabBrowserTabbarDNDObserver(this));
+		return this._tabbarDNDObserver || (this._tabbarDNDObserver = new TreeStyleTabBrowserTabbarDNDObserver(this));
 	},
  
 	get panelDNDObserver() 
 	{
-		delete this.panelDNDObserver;
-		return (this.panelDNDObserver = new TreeStyleTabBrowserTabpanelDNDObserver(this));
+		return this._panelDNDObserver || (this._panelDNDObserver = new TreeStyleTabBrowserTabpanelDNDObserver(this));
 	},
  
 	getCurrentDragSession : function()