Google intentionally slows down YouTube on Firefox by 80% by using old libraries that are unsupported by Firefox because they were deprecated.
https://9to5google.com/2018/07/25/youtube-chrome-firefox-edge-speed-mozilla/
@BurungHantu for the time being, the youtube-classic add-on seems to help https://addons.mozilla.org/en-US/firefox/addon/youtube-classic/
@BurungHantu google is pure evil
@BurungHantu are they still doing this?