Did you try uBlock Origin?

Like TyDraniu mentioned, please try uBlock Origin: https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/ This is a very trustworthy addon. Just be aware that Youtube is attacking all adblockers because they are losing a lot of money by having their ads blocked. Maybe consider turning off the adblocker once in a while to view the ads. I'm doing this also since I started feeling slightly guilty about blocking all their ads. But Youtube does have some pretty long & horrible ads! And whenever I see those, I turn the adblocker back on & don't feel guilty anymore. :)