 <p>Try cleaning up your code a bit, there are a few missing semicolons (<a href="http://www.jslint.com/" rel="nofollow noreferrer">http://www.jslint.com/</a>).</p> <p>I worked with extjs some time ago, and had the same problem with a window rendering in Firefox and not in IE. Maybe IE's JS engine is more sensitive to syntactical errors.</p>