diff --git a/modules/window.js b/modules/window.js
index bbc7bfb4..1a352e76 100644
--- a/modules/window.js
+++ b/modules/window.js
@@ -98,7 +98,7 @@ function TreeStyleTabWindow(aWindow)
 
 	var isDevEdition = this.window.AppConstants.MOZ_DEV_EDITION;
 	if (isDevEdition) {
-		var rootelem = this.document.documentElement;
+		let rootelem = this.document.documentElement;
 		rootelem.setAttribute('treestyletab-devedition', true);
 	}