Turns out this was an artefact of a Firebug bug. The Ajax request was cancelled, but Firebug continued reporting it as active. Details here.
I have long runnin
Calling abort resets the object; the onreadystatechange event handler is removed, and readyState is changed to 0 (uninitialized).
i do not think it cancels the request itself, it just ignores any response (manipulating the event handler)- but i didnt really try it out yet.
but it will deifnitely allow you to reuse the object - no matter if the browser still has some thread working with the old request.