for each...in loop has been virtually removedWhile E4X has been deprecated and disabled from Firefox 17, the for each...in statement, standardized as a part of that, has been left for backward compatibility. In Firefox 20, a change has been made to disable the statement unless Web developers explicitly specify JavaScript 1.6 and later like <script type="text/javascript;version=1.6">. You are recommended to replace with the for...of statement that are standardizing in ECMAScript 6.