main.js

main.js

function showMore() {
  document.getElementById("more-text").hidden = false;
}
alert("heheh");

main.xul

<?xml version="1.0"?>

<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<window id="main" title="My App" width="300" height="300" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

  <script type="application/javascript" src="chrome://myapp/content/main.js"/>

  <caption label="Hello World"/>
  <separator/>
  <button label="More >>" oncommand="showMore();"/>
  <separator/>
  <description id="more-text" hidden="true">This is a simple XULRunner application. XUL is simple to use and quite powerful and can even be used on mobile devices.</description>

</window>

prefs.js

pref("toolkit.defaultChromeURI", "chrome://myapp/content/main.xul");