XMLDocument And XMLHttpRequest ?

I have something like this:

var xmlDoc=document.implementation.createDocument("", "", null);

but in opera xmlDoc.load(...) won't work, so I use:

XMLHttpRequest, and I have all needed code... let's say it works...

So I got serviceRequest.responseXML and is there a way to initialize xmlDoc
in my case ? I am asking because I have code that uses xmlDoc.load(...) in
FF and all code that's need to futher work with XML written. So this would
be comfortable for me to use xmlDoc instead of creating everything from
start.