Dark mode is nice but it doesn't work on some sites like twitch.>>1586
You may like the extension flashgot. It's specifically a download manager, but you can config it to open videos in vlc or mpv if you have youtube-dl installed. So you'd be able to right click a video and send it to your out of browser video player.
Why play videos out of browser? You get better playback and you have the option to user shaders like sharpen or denoise.
Firefox quantum isn't compatible with flashgot because it's in the old extension format so I had to settle for 'Open in VLC'