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.
The business professional
Xmarks (download): If you're running a business and want everyone in the office to have the same resources available to them, Xmarks is the way to do it. Simply download it into your browser, create an account, and install it on all the browsers in the office. Once configured, every Xmarks instance connected to that account will be automatically updated whenever a user adds a bookmark. In other words, if one employee adds Webware to their bookmarks, every other person in the office who's running Xmarks will have it installed, as well.
Jigsaw Contact and Company Search: Most business professionals need to have easy access to a company's contact information. If you're one of those people, consider using the Jigsaw Contact and Company Search. Jigsaw, a global online business directory, features millions of contacts and company profiles that can be searched by title, geography, and business.
Stock Pilot (download): Sometimes going to Google Finance is too time consuming to get quick public company info. Try using Stock Pilot instead. The add-on allows you to switch between financial sites to research a particular stock. It also boasts RSS alerts to update you when news breaks about a company you're tracking. Ready access to SEC filings makes it a handy tool for anyone who wants to see a company's quarterly or annual financial data.
TripSync: Once you install the TripSync travel manager add-on, you'll be blown away by its functionality. You can book and hold reservations to places across the world, receive travel alerts, and place your itineraries on Outlook and Google Calendar. TripSync is designed specifically for the business professional, so it's a bit more useful than using a site like Expedia.
WOT (download): WOT, short for Web of Trust, is designed to ensure security and privacy are maintained while you're browsing. Once installed, the add-on warns you about risky Web sites. It also alerts you to sites that deliver malware or send spam. The add-on shows you safety ratings on over 21 million sites.
1 Stop travel and retail shopping engine: If you want to find the best deals across the U.S., Europe, or Canada, the 1 Stop travel and retail shopping engine add-on is a good place to start. While you're browsing, it allows you to search for flights on more than 40 airlines. It also lets you search for deals on more than 20 sites, including Amazon, eBay, and Wal-Mart. Once you start searching in the add-on, it finds the desired page and automatically opens it in a separate tab.
Buy it online (download): If you've ever been on a Web site and found a product you'd like to buy, but were forced to go to Amazon.com to search for it, you'll be happy to know that there's an add-on designed specifically for you. Dubbed "Buy it online," the add-on adds another option when you right-click on a product title on any Web page. If you click the "Buy it online!" button, you'll be shown a listing of all the stores on the Web that are currently selling the product. All the retailers listed are well-known and trustworthy.
eBayBuddy (download): If you're obsessed with eBay, there's a great add-on you might want to consider. Dubbed eBayBuddy, the tool is available in the right-click menu in Firefox and allows you to have full access to eBay's site map, search, and more. It's a great way to quickly and easily find products on eBay without being forced to deal with the site itself.
KeepCash Coupon Notifier (download): As someone who frequents coupon sites and goes to a retailer's page with the discount ready to be used, I found KeepCash Coupon Notifier to be an extremely useful add-on. Whenever you go to a retail site, it automatically searches its database to find any coupons currently available for the site you're on. If there are, it will list them for you to be used while you're shopping.
PriceAdvance (download): When you run PriceAdvance and surf to a retail site to research a product, the add-on will tell you its price on a handful of other prominent sites across the Web. For instance, if you're on Amazon looking to buy an Xbox 360, the site will automatically search for other retailers selling the console and display their pricing, so you can be sure that you're getting the best price.
The social networker
Delicious Bookmarks (download): If you're a bookmarking fanatic like I am, you won't want to use Firefox without the Delicious Bookmarks add-on. Simply install it and whenever you surf to a page, you can click the Delicious button on your address bar to save it to your Delicious profile with a tag of your choosing. It's simple and a must-have for any Firefox user.
Facebook Toolbar (download): A simple, yet highly useful add-on, the Facebook Toolbar allows you to view friends' statuses, search for people on the popular social network, and interact with friend profiles. It even alerts you when you receive a poke, message, or notification. But perhaps the add-on's most useful feature is the option to share a page you're currently browsing with friends on Facebook. You can either send it to them or add it to your own profile in a few clicks.
GoogleTube: When you use Google Search, it sometimes displays links directing you to a YouTube video that's relevant to your query. But instead of going to the YouTube page, installing GoogleTube gives you the option to watch the video in the Google results page. After installation, the add-on adds a YouTube button next to every YouTube result in Google. When you click it, it plays the respective video next to the results.
Power Twitter (download): If you want Twitter to be more powerful, use Power Twitter. Whether you want to automatically add hashtags to a tweet, expand URLs, add in-line YouTube, Flickr, and TwitPic content, or see a person's status history with a simple mouseover, Power Twitter will do it. Twitter is fun and addicting by itself, but it's much better with Power Twitter installed.
TwitterBar: For those times when you want to update your Twitter profile, but you don't feel like going to your profile page to do it, use TwitterBar. The add-on allows you to update Twitter from Firefox's address bar by typing your message out and clicking a small Twitter icon placed alongside the address bar. If you're worried about the number of characters you have left, you can hover your mouse over the tweet to find out.
Dictionary: What good would a Firefox add-on list be for students if it didn't include a dictionary? With the help of the Dictionary add-on, students won't need to go elsewhere to find the definitions for words. Simply input the word you're looking to define and the add-on will deliver the result.
FacePAD: Just because students are expected to be studying all the time doesn't mean they are. In fact, many of them are on Facebook to check out their friends' pictures. If you're one of those people, install FacePAD. It lets you download anyone's Facebook album or all your friends' albums in just one click.
Google Scholar search bar: Since most students are required to use scholarly publications for research, having a Google Scholar search bar installed in their Firefox installation will certainly come in handy. Instead of going to the Google Scholar page, the add-on lets students search regardless of where they are on the Web. Results pop up in a new tab.
Surf Canyon (downlload): Surf Canyon is a simple add-on that finds relevant information contained in search results on Google, Yahoo, Live Search, Lexis Web, and Craigslist. While it's running in your browser, you can search as you normally would, but Surf Canyon analyzes your query and finds good results as deep as page 100.
Zotero (download): Once students perform their research, they're usually required to cite their sources. And that's where Zotero comes in. The add-on stores PDFs, files, images, and other content for later use. But its best feature is automatic citation exporting, which creates a proper citation for a source that can be sent to a Word or OpenOffice document.