AppTab.xul

<?xml version="1.0"?>
<?xml-stylesheet href="skin.css" type="text/css"?>
<overlay id="AppTab-overlay" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<script type="application/javascript">
<![CDATA[
window.addEventListener('load', function() {
  removeEventListener('load', arguments.callee, false);
  
  function checkforAppTab() {
    if (gBrowser.selectedTab.pinned) {
      document.documentElement.setAttribute('apptabselected','true');
    } else {
      document.documentElement.removeAttribute('apptabselected');
    }
  }

  var container = gBrowser.tabContainer;
  container.addEventListener('TabSelect', checkforAppTab, false);
  container.addEventListener("TabPinned", checkforAppTab, false);  
  container.addEventListener("TabUnpinned", checkforAppTab, false);
}, false);
]]>
</script>
</overlay>