Very high CPU usage in Firefox when watching YouTube on 17.10

Same here, Firefox eats a lot of CPU(>100%) when playing Youtube videos. One tweak is to enter about:config in address bar, then search for below flag and make it true.

media.hardware-video-decoding.force-enabled

After that, the cpu usages becomes 20-40% which is kind of normal and on par with Chromium.

You could possibly get more juice after using this h264ify addon.