Strangely and unfortunately, Firefox doesn't recommend this addon. However, I have used AdBlock Plus for years with no problem, and in the rare cases where you want to turn it off for a website or entirely, it is easy to do. In the rare cases where it fails to block an ad, you can simply have AdBlock Plus block the ad by clicking on the page elements you wish to hide. Can't be easier, and always works. One of my few five-star ratings.

10多年了，谢谢

Reliable for blocking annoying ads and popups!
(from a grateful anonymous beneficiary)