File access using Javascript and XPCOM

I have built a nice UI with XUL and now need to add some functionality. For
example I want to read an an external (remote) XML data file and dynamically
populate a tree widget. (My application will read, modify and save this XML
file eventually).

My question is how to best go about reading in the XML data file. I have
tried using javascript with little success - I used nsIFilePicker to get the
file but I am having trouble passing this to my loading function
(xmlDoc.load). I think this is because this is a "wrapper"???  I am also
concerned about security issues with this approach as well.

Anyway, can anyone help me understand the best approach for this type of
file access and provide an example if possible? I am a newbie so please bear
with me.

Russ