Printing with nsIWebBrowserPrint?

[Kilian Ruess]

I need to print a webpage unattendedly.
I know, nsIWebBrowserPrint.print() can do my job,
but I don't know how ;-(
More precisly I don't know, how to get a component
that implements nsIWebBrowserPrint.

I tried:

<?xml version="1.0"?>
<!DOCTYPE window>
<window
    xmlns:html="http://www.w3.org/1999/xhtml";
    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
    height="600"
    width="800"
    title="Printpreview"
    id="myWindow"
    >
<iframe id="print-target" src="http://www.cool.de"; flex="1"/>
<script>
    var brws =
Components.classes["@mozilla.org/appshell/component/browser/instance;1"].
    var prn = brws.getService(Components.interfaces.nsIWebBrowserPrint);
    prn.print(prn.globalPrintSettings, null);
</script>
</window>

Thanks for your help,
Kilian