Just to verify that your Firefox can handle all the YouTube formats (setting aside the question of speed), do you have blue checkmark boxes for the six test boxes on this page: https://www.youtube.com/html5 Many users report that their computers have no problem when YouTube serves H.264-encoded video, but their CPU gets very busy when YouTube serves VP9-encoded video. You can check the encoding in real time if you right-click the video and choose "Stats for nerds". I don't know what causes it, but one workaround for that issue is this extension: https://addons.mozilla.org/firefox/addon/h264ify/