function browserWindowsCount(expected) {
  var count = 0;
  for (let win of Services.wm.getEnumerator("navigator:browser")) {
    if (!win.closed)
      ++count;
  }
  is(count, expected,
     "number of open browser windows according to nsIWindowMediator");
  is(JSON.parse(ss.getBrowserState()).windows.length, expected,
     "number of open browser windows according to getBrowserState");
}

add_task(async function() {
  browserWindowsCount(1);

  let win = await BrowserTestUtils.openNewBrowserWindow();
  browserWindowsCount(2);
  await BrowserTestUtils.closeWindow(win);
  browserWindowsCount(1);
});