FF3 error when evaluating a cross domain XPath

Hi,

Given a HTML document with frames from two different domains.
I am able to evaluate XPaths across domains in FF2, but FF3 is
throwing an error.

function crossFrameDomainXpath() {
  // Ask for privileges necessary to access other domain's content.
 
netscape.security.PrivilegeManager.enablePrivilege("UniversalBrowserRead");

  // Get document element for the frame with external content.
  var d = top.frames[1].document;

  // evaluate XPath using second frame's document context.
 d.evaluate("//h1/text()", d, null, XPathResult.ANY_TYPE, null);
}

The error I'm getting is
[Exception... "Component returned failure code: 0x80070057
(NS_ERROR_ILLEGAL_VALUE) nsIDOMXPathEvaluator.evaluate" nsresult:
"0x80070057 (NS_ERROR_ILLEGAL_VALUE)" location: "JS frame
http://localhost:3000/alp.html
crossFrameDomainXpath :: line 7" data: no]

I have codebase_principal_support enabled in lieu of signing the
script.
I have confirmed the error on a profile with no extensions, as well as
on Ubuntu64 and Vista64.

I'd greatly appreciate any advice regarding this error.

Andy Watts