nodeValue returns the nodeType value of the current XML node, I believe it returns 1, or 3. Where 1 means it is an XML Element and 3 means it is a text node. Or something along those lines.
firstChild gets the value of the first child inside the parent node. In this case your parent node childNodes[6] , and the firstChild in childNodes[6] would be the text contained within that nodes tag (given there are no more nodes within that childNode).
I am VERY bad at explaining stuff like this, there are a few XML in Flash tutorial at kirupa.com if you want to check them out.