By itself, Firefox is a lean and fast browser (see Firefox 1.0 review), but it lacks some of the functions that we've come to take for granted from Microsoft Internet Explorer. It's easy to remedy that situation with small add-on extensions. Extensions can do loads of tasks, from blocking pop-up ads to playing card games—and even viewing a Web page as if it were in IE—so they can make Firefox a versatile and customizable platform for your browsing activities. But where to start? Which extensions are worth downloading and installing? We evaluated dozens of extensions and present the best 15 for your perusal. It's easy to add them; just go to the Tools | Extensions | Get More Extensions menu to get started.