Very high CPU usage in Firefox when watching YouTube on 17.10
Solution 1:
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.