Out of interest why do you do this? Because your computer struggles to play them in browser? I was doing something similar with vlc in the Flash era because I couldn’t run Flash on my PowerPC Mac running Linux
I do this because it lets me have each video in its own resizeable window, scrub around much more quickly using keyboard/mouse shortcuts, customize many aspects of playback like subtitle styling ,brightness, hue, even flip or rotate the video if I want, set the volume on a per-video basis even >100%, save exactly where I was in a video and how it was set up, not have every video I see a part of show up in my history (even hovering for preview does that now, annoyingly), and install a multitude of plugins to augment the player even further.
Though, I initially did start using mpv for watching videos back in 2014 when I was stuck with a core2duo laptop, and it was the only way of watching 720p or higher video without slowing the machine to a crawl. I'm so used to this setup now that I wouldn't want to go back to having my videos trapped a browser tab. (or an always-on-top firefox popout window)
- Until recently, Firefox did not support VA-API video acceleration on Linux, while mpv supported it for a long time. My last computer struggled to play 1080p videos without hardware acceleration.
- I am used to mpv keybindings, i do not want to use two completely different sets of keybindings for playing local videos and Youtube.
- I like videos in a separate window (this feature Firefox started to support recently as PiP mode).
Does YouTube Premium help with that?