Earlier quoted context omitted.
Do you have details? I noticed that Youtube on Firefox makes the fan spin up on my 2013 rMBP a lot. I currently use Firefox but as a heavy Youtube watcher this is a pretty big negative.
This is because Youtube pushes VP8/VP9-encoded video by default to save on bandwidth. Unfortunately, absence of a hardware decoder consumes a ton of battery. Both Firefox and Chrome receive VP9 encoded video. Safari doesn't understand VP9. So Youtube pushes H.264 video which can be hardware accelerated. To force youtube to send H264 in firefox/chrome you need something like https://addons.mozilla.org/en-US/firefox/ad…
This YouTube page will show which codecs are currently enabled in your browser: https://www.youtube.com/html5