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How to listen to focus event of a window with a browser?
<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<window width="640" height="480"
    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
    onload="">

    <script>
        <![CDATA[
            function onfocus() {
                dump("FOCUSED!n");
            }
            window.addEventListener("focus", onfocus, false);
        ]]>
    </script>

    <browser id="gmail" type="content" src="http://gmail.com/" flex="1" />
    <button label="click here to remove the focus from the browser"/>

</window>