I've noticed this issue lately too. Same situation. Tested with a blank profile, tested with addons disabled. Using the 'stats for nerds' feature, it drops about 300 frames at a time, enough to notice the stutter, yet audio plays. I've not yet noticed this in YouTube Videos themselves, but on YouTube Streams, and also on Netflix where it pauses intermittently. This also doesn't happen in Edge.

With the direction Firefox is going in, and rolling out 'updates' that have now gained a habit of breaking something, somewhere I'm reluctant to click the update button and I'm seriously starting to consider switching Browser after 15 years. It's as though updates don't get properly tested now, they just get pushed almost on the weekly to make Firefox weakly.