// 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,"; let eventPromise = ContentTask.spawn(tab.linkedBrowser, null, function () { return new Promise(resolve => { addEventListener( "DOMDocElementInserted", event => resolve(event.target.documentURIObject.spec), { once: true, } ); }); }); BrowserTestUtils.startLoadingURIString(tab.linkedBrowser, uri); let loadedURI = await eventPromise; is(loadedURI, uri, "Should have seen the event for the right URI"); gBrowser.removeTab(tab); });