I doubt there are restrictions on xhr domains in other browsers. If there were such restrictions, one could get around them easily.

var d = document.createElement("div");
d.innerHTML = "<iframe src=anything-at-all.com></iframe>";
// this fetches the html, just like an xhr request would
d.firstChild.contentDocument.innerHTML;

And I know frames can be inter domain, I see those all the time.

Karl Dahlke