diff --git a/modules/base.js b/modules/base.js
index c9179f9a..5d016fe3 100644
--- a/modules/base.js
+++ b/modules/base.js
@@ -1651,14 +1651,14 @@ var TreeStyleTabBase = inherit(TreeStyleTabConstants, {
 			dump('Tree Style Tab: new child tab is canceled.\n'+
 			     new Error().stack.replace(/^/gm, '  ')+'\n');
 
-		ownerBrowser.treeStyleTab.readiedToAttachNewTab      = false;
-		ownerBrowser.treeStyleTab.readiedToAttachNewTabGroup = false;
-		ownerBrowser.treeStyleTab.readiedToAttachMultiple    = false;
-		ownerBrowser.treeStyleTab.multipleCount              = -1;
-		ownerBrowser.treeStyleTab.parentTab                  = null;
-		ownerBrowser.treeStyleTab.insertBefore               = null;
-		ownerBrowser.treeStyleTab.treeStructure              = null;
-		ownerBrowser.treeStyleTab.shouldExpandAllTree        = false;
+		delete ownerBrowser.treeStyleTab.readiedToAttachNewTab;
+		delete ownerBrowser.treeStyleTab.readiedToAttachNewTabGroup;
+		delete ownerBrowser.treeStyleTab.readiedToAttachMultiple;
+		delete ownerBrowser.treeStyleTab.multipleCount;
+		delete ownerBrowser.treeStyleTab.parentTab;
+		delete ownerBrowser.treeStyleTab.insertBefore;
+		delete ownerBrowser.treeStyleTab.treeStructure;
+		delete ownerBrowser.treeStyleTab.shouldExpandAllTree;
 
 		return true;
 	},
diff --git a/modules/browser.js b/modules/browser.js
index 9a46561b..fb759532 100644
--- a/modules/browser.js
+++ b/modules/browser.js
@@ -3202,7 +3202,7 @@ TreeStyleTabBrowser.prototype = inherit(TreeStyleTabWindow.prototype, {
 			else if (
 				parent &&
 				utils.getTreePref('insertNewChildAt') == this.kINSERT_FISRT &&
-				(this.multipleCount <= 0 || this._addedCountInThisLoop <= 0)
+				(typeof this.multipleCount !== 'number' || this._addedCountInThisLoop <= 0)
 				) {
 				/* 複数の子タブを一気に開く場合、最初に開いたタブだけを
 				   子タブの最初の位置に挿入し、続くタブは「最初の開いたタブ」と