My top three that I cannot live without:
- https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/ Adblock Plus - which going by the above, everyone is aware of
- https://www.eff.org/https-everywhere HTTPSEverywhere - this extention was developed by the Eletronic Frontier Foundation and forces around 200 websites to use the secure version of the website as opposed to the unsecure version.
- http://www.qfxsoftware.com/ KeyScrambler - the free version is all you need for the browser. Basically what it does is prevent keyloggers from getting your passwords, etc. A must use for anyone who does online banking.