How to prevent pop-up blocking in Firefox

URLUtil.as

package {

    import flash.external.ExternalInterface;

    public class URLUtil    {

        protected static const WINDOW_OPEN_FUNCTION : String = "window.open";

        public static function openWindow(url : String, window : String = "_blank", features : String = "") : void {
            ExternalInterface.call(WINDOW_OPEN_FUNCTION, url, window, features);
        }
    }
}