I am working on a Firefox extension, and in that extension I am trying to use AJAX to submit a form on a webpage. I am using:

var request = Components.classes["@mozilla.org/xmlextras/xmlhttprequest;1"].createInstance(Components.interfaces.nsIXMLHttpRequest);
request.onload = loadHandler;
request.open("POST", url, true);
request.send(values);

to make the request, and it works ... mostly. The one problem is that the form has an authentication token on it, and I need to submit that token with my POST. I tried doing a GET separately to get this token, but by the time I made my second (POST) request my session had (evidently) changed, and the authenticity token was considered invalid.

Does anyone know of a way to use the XUL/Chrome Javscript to maintain a constant session across multiple requests (all "behind the scenes") for something this? I'm still a XUL n00b, so there may be a totally obvious alternative that I'm missing (eg. hidden IFRAME; I tried that briefly but couldn't get it to work).