fune/netwerk/test/browser/browser_test_favicon.js

// Tests third party cookie blocking using a favicon loaded from a different
// domain. The cookie should be considered third party.

add_task(async function() {
  const iconUrl =
    "http://example.org/browser/netwerk/test/browser/damonbowling.jpg";
  const pageUrl =
    "http://example.com/browser/netwerk/test/browser/file_favicon.html";
  await SpecialPowers.pushPrefEnv({
    set: [["network.cookie.cookieBehavior", 1]],
  });

  let promise = TestUtils.topicObserved("cookie-rejected", subject => {
    let uri = subject.QueryInterface(Ci.nsIURI);
    return uri.spec == iconUrl;
  });

  // Kick off a page load that will load the favicon.
  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, pageUrl);
  registerCleanupFunction(async function() {
    BrowserTestUtils.removeTab(tab);
  });

  await promise;
  ok(true, "foreign favicon cookie was blocked");
});