h264ify is a Firefox/Chrome extension that makes YouTube stream H.264 videos instead of VP8/VP9 videos.
Try h264ify if YouTube videos stutter, take up too much CPU, eat battery life, or make your laptop hot.
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.
Looking for the Chrome version? See: https://github.com/erkserkserks/h264ify
Install from here: https://addons.mozilla.org/firefox/addon/h264ify/
If all goes well, when you visit https://www.youtube.com/html5, you should see this:
Note: The current version of Firefox (35) doesn't support MSE, but when support arrives, h264ify will also block VP9.