Using XPath namespaces in Firefox

I am attempting to parse an xml file in Javascript while ignoring the namespace of the xml file. I don't anticipate the namespace of the xml file changing in the future, though I'd be happier if the code would work even if the namespace did change arbitrarily. I attempted to follow these instructions unsuccessfully.

What do I need to change so that my second version successfully finds the xml node? Below is my code, which does not work as expected. Speci开发者_JAVA技巧fically, the third call to Components.utils.reportError outputs null).

Javascript (in a firefox extension):

Components.utils.reportError("XML A: " + docky.documentElement);
var testxpath = '//' + docky.documentElement.tagName + ":entry";
Components.utils.reportError("Test XPath: " + testxpath);
Components.utils.reportError(docky.evaluate(testxpath, docky, 
    function(){return 'http://tempuri.org/';}, 
    XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue);

XML File:

<OutputBlob xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
        xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        xmlns="http://tempuri.org/">
    <rnd>
        <double>0.23500252665719135</double>
        <double>0.82559380951597994</double>
        <double>0.33431208335529644</double>
        <double>0.91389494431852125</double>
    </rnd>
</OutputBlob>

Output:

XML A: [object Element]
Test XPath: //OutputBlob:entry
null

---

The variation on my code found below (i.e., where namespaces are not involved) works as expected. That is, the third call to Components.utils.reportError does not output null.

Javascript (in a firefox extension):

Components.utils.reportError("XML A: " + docky.documentElement);
var testxpath = '//' + docky.documentElement.tagName;
Components.utils.reportError("Test XPath: " + testxpath);
Components.utils.reportError(docky.evaluate(testxpath, docky, null, 
    XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue);

XML File:

<OutputBlob xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:xsd="http://www.w3.org/2001/XMLSchema" >
    <rnd>
        <double>0.23500252665719135</double>
        <double>0.82559380951597994</double>
        <double>0.33431208335529644</double>
        <double>0.91389494431852125</double>
    </rnd>
</OutputBlob>

Output:

XML A: [object Element]
Test XPath: //OutputBlob
[object Element]

---

Fix: Replace

var testxpath = '//' + docky.documentElement.tagName + ":entry";

with

var testxpath = '//entry:' + docky.documentElement.tagName;