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
