http://www.xulplanet.com/tutorials/xultu/events.html http://www.xulplanet.com/reference/elemref/ref_script.html Much like the HTML script element, this is used to declare a script to be used by the XUL window. You should usually put scripts in a separate file pointed to by the src attribute, but you may also place the script inline inside the opening and closing script tags. The URL of the script. The language of the script. Usually, you would set this to 'application/x-javascript'.