How it works
From their website:
“Makes YouTube stream H.264 videos instead of VP8/VP9 videos
By default, YouTube streams VP8/VP9 encoded video. However, this can cause problems with less powerful machines because VP8/VP9 is not typically hardware accelerated.
In contrast, H.264 is commonly hardware accelerated by GPUs, which usually means smoother video playback and reduced CPU usage. h264ify makes YouTube stream H.264 videos instead of VP8/VP9 videos.”
I have been using it for a while and in addition to being higher-performance, it also resolved some screen-tearing issues I was having with youtube video playback on linux. I highly recommend it.
Firefox Add-On: https://addons.mozilla.org/en-US/firefox/addon/h264ify/
Google Chrome Extension: https://chrome.google.com/webstore/detail/h264ify/aleakchihdccplidncghkekgioiakgal