External node troubles [FF3 RC1]

Note: This is for FF3 RC1

I'm running an XMLHttpRequest against a site, any site, and then
parsing the contents with DOMParser and XMLSerializer.  I have the
following code

// html is the responseText from my XHR
$('test').adoptNode = html.split(/<body[^>]*>((?:.|\n)*)<\/body>/i)
[1]; // xmlserializer needs a dom object
var p = new DOMParser(), xs = new XMLSerializer();

var doc = p.parseFromString(xs.serializeToString($('test')),"text/
xml");
var div = document.createElement('div');

var alt = $x("//IMG/@alt", doc); // result: Node cannot be used in a
document other than the one in which it was created
var alt1 = $x("//IMG/@alt", document.adoptNode(doc)); // result:
Operation is not supported (NS_ERROR_DOM_NOT_SUPPORTED_ERR)

function $(item) {return document.getElementById(item);}

function $x(p, context) {
  if (!context) context = document;
  var i, arr = [], xpr = document.evaluate(p, context, null,
XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
  for (i = 0; item = xpr.snapshotItem(i); i++) arr.push(item);
  return arr;
}

Help on these two errors?