It will show in your browser, firefox directions, Tools/Addons, a windows opens then select extensions. If you want to see if it's still there.
As for a site working, just click on the stop sign looking icon on your browser frame, and tell it to disable on your troubled sites. You can set it for page specific or entire site.
I use Adblock Plus, and Adblock Pop-up Addon, it really works great if you use both of them together.
And if that doesn't cut out enough stuff for you try Ghostery, it will make a few things not work at first, but you can easily figure out which things need to be enabled . It removes all tracking from your browser, you would be amazed at how much your tracked on any search. They claim it's how they target you for ads.