gecko-dev/dom/tests/unit/test_xhr_init.js

function run_test() {
  var x = new XMLHttpRequest({ mozAnon: true, mozSystem: false });
  Assert.ok(x.mozAnon);
  Assert.ok(x.mozSystem); // Because we're system principal

  x = new XMLHttpRequest({ mozAnon: true });
  Assert.ok(x.mozAnon);
  Assert.ok(x.mozSystem);

  x = new XMLHttpRequest();
  Assert.ok(x.mozAnon);
  Assert.ok(x.mozSystem);

  Services.prefs.setBoolPref(
    "network.fetch.systemDefaultsToOmittingCredentials",
    false
  );
  x = new XMLHttpRequest();
  Assert.ok(!x.mozAnon);
  Assert.ok(x.mozSystem);
}