diff --git a/content/treestyletab/windowHelper.js b/content/treestyletab/windowHelper.js
index 28e92c31..6bf2ad74 100644
--- a/content/treestyletab/windowHelper.js
+++ b/content/treestyletab/windowHelper.js
@@ -75,8 +75,12 @@ var TreeStyleTabWindowHelper = {
 		var strip = this.service.getTabStrip(aObserver) ||
 					gBrowser.mStrip // fallback to the default strip, for Tab Mix Plus;
 
-		if (aObserver.tabContainer &&
-			aObserver.tabContainer.tabbrowser == aObserver) { // Firefox 4.0 or later
+		// Firefox 4.0 or later
+		if (
+			aObserver.tabContainer &&
+			aObserver.tabContainer.tabbrowser == aObserver &&
+			this.service.isGecko2 // tabbar.tabbrowser can be defined by addons like Tab Mix Plus.
+			) {
 			aObserver = aObserver.tabContainer;
 		}
 
diff --git a/modules/browser.js b/modules/browser.js
index 3d4632cf..cc494329 100644
--- a/modules/browser.js
+++ b/modules/browser.js
@@ -974,6 +974,9 @@ TreeStyleTabBrowser.prototype = {
 			scrollFrame = d.getAnonymousElementByAttribute(b.mTabContainer, 'class', 'tabs-frame') ||
 							d.getAnonymousElementByAttribute(b.mTabContainer, 'anonid', 'scroll-tabs-frame');
 			newTabBox = d.getAnonymousElementByAttribute(b.mTabContainer, 'id', 'tabs-newbutton-box');
+			let newTabButton = document.getElementById('new-tab-button');
+			if (newTabButton && newTabButton.parentNode == b.tabContainer._container)
+				newTabBox = newTabButton;
 			tabBarMode = this.getPref('extensions.tabmix.tabBarMode');
 		}
 
@@ -1017,7 +1020,8 @@ TreeStyleTabBrowser.prototype = {
 				d.getAnonymousNodes(scrollFrame)[0].removeAttribute('flex');
 				scrollFrame.parentNode.orient =
 					scrollFrame.orient = 'vertical';
-				newTabBox.orient = 'horizontal';
+				if (newTabBox)
+					newTabBox.orient = 'horizontal';
 				if (tabBarMode == 2)
 					this.setPref('extensions.tabmix.tabBarMode', 1);
 			}
@@ -1105,7 +1109,8 @@ TreeStyleTabBrowser.prototype = {
 				d.getAnonymousNodes(scrollFrame)[0].setAttribute('flex', 1);
 				scrollFrame.parentNode.orient =
 					scrollFrame.orient = 'horizontal';
-				newTabBox.orient = 'vertical';
+				if (newTabBox)
+					newTabBox.orient = 'vertical';
 			}
 
 			if (toolboxContainer)