add xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"to tag <html>.
then use <xul:element>for example.<xul:vbox>
Edit
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
</head>
<body>
<xul:vbox>
</xul:vbox>
</body>
</html>
Also, I guess this is not such a simple case ... otherwise there would not be much point in wrapping the xul in html (although sometimes this happens sometimes)
Edit
Some additional points to consider when doing this:
( )