gecko-dev/browser/base/content/test/general/browser_bug479408.js

function test() {
  waitForExplicitFinish();
  let tab = (gBrowser.selectedTab = BrowserTestUtils.addTab(
    gBrowser,
    "http://mochi.test:8888/browser/browser/base/content/test/general/browser_bug479408_sample.html"
  ));

  BrowserTestUtils.waitForContentEvent(
    gBrowser.selectedBrowser,
    "DOMLinkAdded",
    true
  ).then(() => {
    executeSoon(function () {
      ok(
        !tab.linkedBrowser.engines,
        "the subframe's search engine wasn't detected"
      );

      gBrowser.removeTab(tab);
      finish();
    });
  });
}