firefox3 xpath

Dear All,

I was using firefox2 to load the following script with no problem,

var xmlDocNode;
var xml = loadXMLDoc(xmlStr); //transform to xml document from string
xmlDocNode = xml;
var nodes = document.evaluate(path, xmlDocNode, null,
XPathResult.ANY_TYPE,null);

but when I shift to firefox3 it create WRONG_DOCUMENT_ERR, and after
some search, I found that I have to use document.importNode or
document.adoptNode to import a document from external source since it
has changed in firefox3. So I changed my code to,

var xmlDocNode;
var xml = loadXMLDoc(xmlStr); //transform to xml document from string
xmlDocNode = docuemnt.importNode(xml, true);
var nodes = document.evaluate(path, xmlDocNode, null,
XPathResult.ANY_TYPE,null);

but then it raise another error, Operation is not supported.
Any ideas??

Thanks