Hover a thumbnail on youtube.com and click an icon at the right: "Not interested" and "Don't recommend channel"
< Feedback op YT: not interested in one click
It works for me, so I can't fix what ain't broke. Either youtube is serving a different site randomly for people or you have an interfering extension/userscript, so try my script in a new profile with nothing else.
Not working with last update :(
Firefox 109.0.1 x64
Win 10