Hover a thumbnail on youtube.com and click an icon at the right: "Not interested" and "Don't recommend channel"
< YT: not interested in one clickについてのフィードバック
Never mind, just went back to youtube and it's working the same. No idea what the hell was going on, but it's good now.
This is one of two scripts that make youtube usable for me. Unfortunately it stopped working over night. I'm running Firefox 92.0a1 (2021-08-04) (64-bit). Icons just don't show up.
I marked it "bad" but this is a fantastic addition that I had been using for months and months.