All security add-ons are here (like AddBlock Plus, NoFlash and so on)
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
Protect your privacy. See who's tracking your web browsing and block them with Ghostery.
Find out which websites you can trust. WOT adds intuitive traffic light-style icons next to search results and URLs to help you make informed decisions about whether to visit a site or not.
The quickest way to find out if your computer is infected!
Bitdefender QuickScan is a very fast antivirus scanner, able to determine in a matter of seconds if a system is infected with malware.
More on http://quickscan.bitdefender.com/faq
BlockSite is an extension, which automagically blocks websites of your choice. Additionally, this extension will disable all hyperlinks to these websites, by just displaying the link text without the clicking functionality.
Protect your Passwords, Payments, and Privacy.
Lightbeam is a Firefox add-on that enables you to see the first and third party sites you interact with on the Web. Using interactive visualizations, Lightbeam shows you the relationships between these third parties and the sites you visit.
Be in control of which cross-site requests are allowed. Improve the privacy of your browsing by not letting other sites know your browsing habits. Secure yourself from Cross-Site Request Forgery (CSRF) and other attacks.
This extension provides various customization options for the Adblock Plus user interface.
Would you like to know which websites are safe and which website you can trust?
The Webutation add-on shows you the reputation of a site with a simple green/yellow/red icon and scans websites against virus and bad userfeedback. Works similar to WOT.
More Advanced (hidden) configures
An artware browser add-on to protect privacy in web-search. By issuing randomized queries to common search-engines, TrackMeNot obfuscates your search profile(s) and registers your discontent with surreptitious tracking.