My test video is https://www.youtube.com/watch?v=lEuHOFgjrAo watched at 1080p60 in full screen. It's a reasonably good test of 60fps smoothness, though synthetic scrolling tests like TestUFO screen recordings might be better strictly speaking.

- 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).