How to tell Firefox NOT to do DOM-Tree creation

Hi,

I try to create an extension for Fitrfox that does something with a
requested XML document.
I want to pass the whole content of the document to a function, so I
don't need the DOM tree.

Since I will deal with very large XML documents it would be annoying
if FF would do the DOM tree creation.

How can I tell Firefox within my extension NOT to do the DOM-Tree
creation?