Adblock Plus

http://adblockplus.org/

https://addons.mozilla.org/en-US/firefox/addon/1865

see Ars' article from the prospective of a content provider

http://arstechnica.com/business/news/2010/03/why-ad-blocking-is-devastating-to-the-sites-you-love.ars

To turn it off, you can disable it in the pop up or use Firefox's Add-ons manager

What you may missed if it is enabled - splash, banner, sidebar ads. Click on its icon to see how many items were blocked.

Choose Open blockable items to see details

To block images, objects like flash not currently block

Preferences