fantastic extension for Firefox all the security extensions.
AdBlock Plus - remove most advertisements from websites.
Cert Viewer Plus - Provides enhancements to the certificate viewer.
Dr. Web Anti-Virus link checker - Great for the initial check before downloading something from the Internet.
Extended Cookie Manager - Manage cookie permissions of websites with ease.
Flashblock - Blocks Flash content on websites with the option to enable it.
FoxTor - Use the Tor network when surfing with Firefox. Adds anonymity.
No Script - Run Java, Javascript and other content only from trusted websites. You create a list of trusted websites that are loaded with all those scripts enabled. All other sites are run without.
Keyscrambler Personal - Encrypts your keystrokes which prevents that keyloggers can record the entered data.
Secure Login - If you want to save usernames and passwords you should install Secure Login. It provides similar functionality to Opera’s Wand feature by bypassing the forms on websites and sending the login data directly to the server.
Whois Plugin - Find out who owns a website.