xhr-context-1725.user.js

// ==UserScript==
// @name        XHR Context
// @namespace   http://github.com/arantius
// @include     http*
// @version     1
// ==/UserScript==

// See: https://github.com/greasemonkey/greasemonkey/issues/1725

(function() {
  // Note that "r" lives in this anonymous function's private scope!
  var r = Math.random();
  console.log('Starting request, r =', r);
  GM_xmlhttpRequest({
    'method': 'GET',
    'url': '/',
    'onload': onResponse,
    'context': r,
  });
})();

function onResponse(response) {
  // But here onResponse can read it via the context property.
  console.log('Request complete; context =', response.context);
}