If you're a Firefox user, chances are you've been using add-ons to extend its functionality. But if you haven't, or if you're looking for new add-ons, I've found 20 for you to try. But since installing all 20 will probably slow your browser to a crawl, I've broken them into four categories. I have grouped the add-ons into "packs" for business professionals, shoppers, social-network fanatics, and students.