possible bug in XMLHttpRequest

Hello,
I am trying to get remote XML with XMLHttpRequest. my source code is:

function load() {
   var req = new XMLHttpRequest();
   req.open("GET","http://www.foo.com/anxml.xml",false);
   req.setRequestHeader('content-type','text/xml');
   req.onload = function() {
     alert("received");
   }
   req.send(null);

}

it works, but When I execute the function load more than once, the 
memory used by mozilla grows exactly the size of the document I am 
requesting. For example:

first time: 22.4 mb;
second time: 23.4 mb;
third time: 24.4 mb;

when the file size is 1mb;

But, if I initialize the req object like a global variable, the memory 
is always the same.

I don't understand why the memory is being increased. The object would 
be destroyed every time the function load finishes. Or it would not?

Is it a bug? Is it correct?

Thanks.