xml DOM difference between IE and Firefox

Say I have an XML string that looks something like this:

<users>
   <user>
     <option value="foo">bar</option>
     <option value="foo2">bar2</option>
   </user>
   <user2>
     <option value="foo">bar</option>
     <option value="foo2">bar2</option>
  </user2>
</users>

I can load this into an XML doc on IE like this:

    txt = ... // xml string
    xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
    xmlDoc.async = false;
    xmlDoc.loadXML(txt);

On Firefox, like this:

    var parser = new DOMParser();
    xmlDoc = parser.parseFromString(txt, "application/xml");

So far, everything is good.

When I want to extract parts of the document, on IE I can do this:

xmlObjs = xmlDoc.getElementsByTagName("user/option");

This gives me a collection of the two options under user.

However, it does not work on Firefox - I get nothing.

I can do this:

xmlObjs = xmlDoc.getElementsByTagName("option");

but this gives me 4 nodes - two under user, and two under user2.

I could do this:

xmlObjs = xmlDoc.getElementsByTagName("user");

and walk the children looking for those nodes with a value of "option", but this takes a lot more code.

How do I do

xmlObjs = xmlDoc.getElementsByTagName("user/option");

in Firefox? I am converting a large web app to be able to run on both IE and Firefox, and the above line is in many places, I'm trying to minimize the changes I have to make.

Thanks.