There's more to do actually, now
go to About:config
find media.mediasource.enabled and toggle it to false
find media.cache_readahead_limit and change it to 9999
find media.cache_resume_threshold and change it to 9999
additionally if you'd prefer mp4 to webm
also in about:config, find:
media.encoder.webm.enabled
media.mediasource.webm.audio.enabled
media.mediasource.webm.enabled
media.webm.enabled
and toggle them all to false
note!
this will limit video to 1080p
and use https://addons.mozilla.org/en-US/firefox/addon/dont-accept-webp/ to kill WebP
Fuck Google