When I open a youtube video in firefox, what I get is a lot of frame drop, visible in the “stats for nerds”. Just tried with a random YT 4k video fullscreen, and it has around 70% framedrop. That feels to me like not flawless playing.
Opening the same video within smplayer with x11 output and YT internal support shows under “Display” “Dropped Frames: 0 (decoder) 0 (output)”.
So for me the viewing in smplayer is flawless (which is I guess a subjective feeling). And not flawless is for me was related to framedrop in other players.
I am actually happy with this setup, that’s why I wanted to share it in case it helps someone else, because the other hints on this forum were not working as good as this one for me.