diff --git a/content/treestyletab/windowHelper.js b/content/treestyletab/windowHelper.js
index a68005ee..50ca7eb8 100644
--- a/content/treestyletab/windowHelper.js
+++ b/content/treestyletab/windowHelper.js
@@ -259,7 +259,7 @@ var TreeStyleTabWindowHelper = {
 			return FeedHandler.__treestyletab__loadFeed.call(this, aHref, aEvent);
 		};
 
-		if ('showNavToolbox' in FullScreen) { // for Firefox 40 or later
+		if ('showNavToolbox' in FullScreen) {
 			FullScreen.__treestyletab__showNavToolbox = FullScreen.showNavToolbox;
 			FullScreen.showNavToolbox = function(...aArgs) {
 				var beforeCollapsed = this._isChromeCollapsed;
@@ -278,16 +278,6 @@ var TreeStyleTabWindowHelper = {
 				return retVal;
 			};
 		}
-		else if ('mouseoverToggle' in FullScreen) { // for Firefox 39 or older
-			FullScreen.__treestyletab__mouseoverToggle = FullScreen.mouseoverToggle;
-			FullScreen.mouseoverToggle = function(...aArgs) {
-				var beforeCollapsed = this._isChromeCollapsed;
-				var retVal = FullScreen.__treestyletab__mouseoverToggle.call(this, ...aArgs);
-				if (beforeCollapsed !== this._isChromeCollapsed)
-					gBrowser.treeStyleTab.updateFloatingTabbar(gBrowser.treeStyleTab.kTABBAR_UPDATE_BY_FULLSCREEN);
-				return retVal;
-			};
-		}
 
 		FullScreen.__treestyletab__toggle = FullScreen.toggle;
 		FullScreen.toggle = function(...aArgs) {