this effectively extracts media urls from the current page, puts them into a m3u file and let's you open it with any application https://addons.mozilla.org/en-US/firefox/addon/play-with/

mpv itself uses ytdl (or others) for accessing streming sites and will buffer the whole vid if you crank up cache settings like:

  cache=yes
  demuxer-max-bytes=5G
  demuxer-max-back-bytes=5G
  prefetch-playlist=yes
  force-seekable=yes