Modification of a XMLDocument root

Hi

I am writing a Firefox extension and I need to display a XML Document 
created from a XML String.
I do not want to create a file that contains this XML String and to load 
this file.
Thus my idea (the only one I found) is to have a simple XML file in my 
extension, to load it via a chrome address and then to modify the 
XMLDocument.
The code looks like:
window._content.document.removeChild(window._content.document.documentElement);
window._content.document.appendChild(aNewXmlElement);

However, the page is empty.
With a HTMLDocument, I can set the body attribute or I can use the 
document.write(...) method. But this attribute and this method do not 
exist for a XMLDocument.

Any ideas ?