Firefox already has several builds made to test out experimental features, but now you can take some of them for a spin even if you only have the stable browser. Mozilla has re-launched Test Pilot for Firefox, an add-on it first released in 2009 to study how people interacted with their browser. This one comes with three features the team is still in the midst of developing. First is Activity Stream, which turns every new tab into a home-page-slash-news-feed that displays the newest posts from your top websites. It also highlights some of the pages you've bookmarked.