mozilla-release/layout/forms/test/bug665540_window.xhtml

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin" type="text/css"?>

<window title="Test Select Dropdown Positioning in Fullscreen Window"
        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        sizemode="fullscreen">

  <script>
    window.arguments[0].SimpleTest.waitForFocus(window.arguments[0].childFocused, window);
  </script>

  <body xmlns="http://www.w3.org/1999/xhtml">
    <select id="select" style="-moz-appearance:none">
      <option id="optiona">a</option>
      <option>b</option>
    </select>
  </body>
</window>