gecko-dev/dom/html/test/browser_DOMDocElementInserted.js

// Tests that the DOMDocElementInserted event is visible on the frame
add_task(async function() {
  let tab = BrowserTestUtils.addTab(gBrowser);
  let uri = "data:text/html;charset=utf-8,<html/>";

  let eventPromise = ContentTask.spawn(tab.linkedBrowser, null, function() {
    const { PromiseUtils } = ChromeUtils.import(
      "resource://gre/modules/PromiseUtils.jsm"
    );
    let deferred = PromiseUtils.defer();

    let listener = event => {
      removeEventListener("DOMDocElementInserted", listener, true);
      deferred.resolve(event.target.documentURIObject.spec);
    };
    addEventListener("DOMDocElementInserted", listener, true);

    return deferred.promise;
  });

  BrowserTestUtils.loadURI(tab.linkedBrowser, uri);
  let loadedURI = await eventPromise;
  is(loadedURI, uri, "Should have seen the event for the right URI");

  gBrowser.removeTab(tab);
});