Essential Rated 5 out of 5 stars
Adblock Plus is a highly manageable app which helps you avoid the vast majority of online advertising. Now some sites do need to display ads and ask that you disable Adblock, which is fair enough. But that is YOUR choice. If you choose to use those sites whose development depends on ad money, then you can easily disable AdBlock. What about more important sites, such as banks or shopping? Rule of thumb: disable AdBlock on all such sites to ensure that everything runs smoothly. Obviously if you don't trust your bank's website, you should bank elsewhere :).
But think of those tiring ads on popular YouTube channels. No problem: AdBlock ensures you do not have to watch those ads. You can even choose to block specific ads that may still be displaying on a site. Customization is incredible and not only will you enjoy a mostly ad-free online life but you can definitely improve your browser's security at the same time.
I consider AdBlock one of the two essential apps for Firefox (the other one being NoScript). These two apps are the reason I use Firefox.