YouTube warns it might make your viewing experience worse if you don't turn off your ad-blocker
(www.businessinsider.com)
If for some reason you can't use a full-on ad blocker, "SponsorBlock for Youtube" is another Firefox extension you can install. It doesn't block ads per se, but it will auto-skip segments of a video that have been reported as ad content. Kind of a grassroots thing, like reporting cops on Waze.