Jason Meserve has added on every add-on and plugged in every plug-in. If you're ready to take your browsing to the next level, he's got a list of 10 add-ons for Mozilla's Firefox that will do just that.
Check out the full list here.
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Web of Trust
Another great add-on you might want to try is Web of Trust, a people-driven security tool that warns Internet users about dangerous and suspicious websites. Site reputation data comes from the user community in combination with trusted sources such as listings of phishing sites.
WOT is a recommended add-on from Mozilla. https://addons.mozilla.org/en-US/firefox/addon/3456