To be honest, that could be just be some interaction with Waterfox specifically. It's not the first time I have encountered people having various issues with that firefox fork that other browsers don't encounter.
- Tried creating a new blank Waterfox profile with no changes except uBlock0. It drops frames, and the borders of the video (4:3 pillarboxes on a 16:9 screen) have a blue or green tint after I seek.
- Same result in a new blank Firefox ESR 115.6.0 (last Windows 7 release) profile with uBlock0.
- The YouTube embed player at https://www.youtube.com/embed/lEuHOFgjrAo, on my main Waterfox profile, runs as smoothly as Invidious (no stutters unless I'm interacting with the player UI).