Are they? I watch YouTube on Firefox all the time, seems fine on my machine.
I think maybe 5+ years ago there were some performance issues caused by YT relying on features that were only implemented in Chrome, but I don’t recall having any issues wrt that for years.
Kinda curious how they picked these. Not that any of these are great, but most pg-13 horror movies are so forgettable it seems the author just overlooked them.