Updated version

iframe.js

let { Cc, Ci } = require('chrome');

let appShellService = Cc['@mozilla.org/appshell/appShellService;1'].
                        getService(Ci.nsIAppShellService);
let service = Cc["@mozilla.org/observer-service;1"].
                getService(Ci.nsIObserverService);
                
let window = appShellService.hiddenDOMWindow;
let document = window.document;
let XUL = 'http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul'

let frame = document.createElementNS(XUL, "browser");
frame.setAttribute("type", "content");
frame.setAttribute("src", "data:;charset=utf-8,");

document.documentElement.appendChild(frame);

let docShell = frame.docShell;
docShell.allowAuth = true;
docShell.allowJavascript = true;
docShell.allowPlugins = true;

// New platform capability is required to add window control
// For details see: Bug 635673
if ("allowWindowControl" in docShell)
  docShell.allowWindowControl = false;
  
frame.setAttribute("flex", 1);
frame.setAttribute("transparent", "transparent");
frame.setAttribute("showcaret", true);
frame.setAttribute("clickthrough", "always");
frame.setAttribute("autocompleteenabled", true);
frame.setAttribute("autoscroll", true);

frame.setAttribute("src", "data:text/html,<script>" +
                          "setTimeout(function(){" +
                             "window.location='about:blank'" +
                          "},450);" +
                          "</script>");

frame.addEventListener("load", function(event) {
  console.log("observe destruction of: ", event.target.URL);
  
  let observeID = frame.contentWindow.
                  QueryInterface(Ci.nsIInterfaceRequestor).
                  getInterface(Ci.nsIDOMWindowUtils).
                  currentInnerWindowID;

  let observer = {
    observe: function(subject, topic, data) {
      let windowID = subject.QueryInterface(Ci.nsISupportsPRUint64).data;
      console.log("observe: ", observeID, " window: ", windowID)
      if (windowID === observeID) {
        console.log("<<<< unload >>>>")
        service. removeObserver(observer, "inner-window-destroyed");
      }
    }
  }

  service.addObserver(observer, "inner-window-destroyed", false);
}, true)

output.txt

info: scratch-kit: observe destruction of:  data:text/html,<script>setTimeout(function(){window.location='about:blank'},450);</script>
info: scratch-kit: observe destruction of:  about:blank