fune/browser/base/content/test/general/browser_bug537474.js

add_task(async function() {
  let browserLoadedPromise = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
  window.browserDOMWindow.openURI(makeURI("about:"), null,
                                  Ci.nsIBrowserDOMWindow.OPEN_CURRENTWINDOW, null,
                                  Services.scriptSecurityManager.getSystemPrincipal())
  await browserLoadedPromise;
  is(gBrowser.currentURI.spec, "about:", "page loads in the current content window");
});