Update as of 2023: This add-on is now depreciated.
FireFox has tons of cool addons and here is one more.
This is a TOR add-on which works using TOR(The Onion Ring) network.The reason it is called onion is because it involves the use of many layers through which the end user data is routed anonymously before reaching the end point.
What this simply means is that there is no definite way to monitor or hijack the end user data since so many layers are involved(the routers through which data is bounced are called onion routers ;)).
Once the add-on is installed,you can change the default preferences of allowing/blocking various stuff as follows :
Go to Tools > Add-ons in Firefox and select Preferences for Torbutton
Web pages may load a bit slower because of the network data bouncing off so many routers.
Just install the add-on and start surfing the net all anonymously 🙂