Absolutely essential browsing for Firefox users. While the basic browser is great, the addons make it unique to you and your needs.
Check out, especially: Adblock plus, Autopager, Greasemonkey, WOT (Web Of Trust), Downloadhelper, Flashblock, Flashgot, Greasefire, Hyperwords, Fire Gestures, TooManyTabs ... oh and about a hundred more, just to start with.
If you install nothing else, Adblock Plus is an absolute must, removing all manner of ads from pages forever and allowing you to remove even more as you go along.
For the six people out there on the web who still haven't a clue what I'm on about, Firefox addons add some kind of new functionality to the browser. They are free, and several are recommended by the Mozilla community so you can stick to those if you don't want to get experimental. Some of them are truly awesome in the significance of the feature they add (e.g. Adblock Plus), some do a job that makes life a little easier (e.g. Autopager), and some perform useful housework (e.g, FEBE which backs up your entire FF environment).
The ability to see where a site is really coming from, and to find out much of the public information on it, is provided by an addon called FlagFox. It doesn't appear to do much, and just pops a little flag into your address bar, but boy, it's handy to know if that electronics dealer is in China or not. A click provides geographical data about the server and the IP owner if that information is available, and there's a WHOIS lookup as well.
DT Whois also provides an immediate WHOIS lookup on the Domaintools site. There are many other addons that fetch whois information and/or perform some useful internet tool functions if you're the sort of person that needs them.
If you're a web developer, or simply don't want sites to see where you're coming from, the Web Developer Toolbar is awesome, allowing you to see all the various elements that make up a web page and turn them on and off. Tick "disable referrers" in the first menu, and when you go to a new page, the browser won't remember where it's been. Validate your code, examine cookies and heaps more.
Once you start in on addons, you'll be hooked and popping back every now and again to see the new ones, or experiment with older ones you haven't got around to yet. Some are truly experimental and out on the cutting edge (i.e. they crash) and others are mature and used by hundreds of thousands of FF users. There's always another one to play with, and you can load and unload them as you see fit, there's no commitment to installing anything you don't like.
And the SiteJabber admins will be delighted if, as a result of reading this, you install the SiteJabber addon too!