Since the HTML <a> tag is not working in XUL we need another approach to open a URL.
You can use the function below in your project:

function gotoUrl(url)
{
var nsioservice= Components.classes["@mozilla.org/network/io-service;1"]
.getService(Components.interfaces.nsIIOService);

var uri = nsioservice.newURI(url, null, null);

var eps = Components.classes["@mozilla.org/uriloader/external-protocol-service;1"]
.getService(Components.interfaces.nsIExternalProtocolService);

eps.loadURI(uri, null);
}

Use it like this:

gotoUrl(‘http://www.short-tag.info’);