Firefox Slow

I would try to backup your profile and reset firefox using about:support, that should fix your problems, I do it from time to time and does wonders.

If that doesn't work, I can suggest 3 settings, that are experimental, but may speedup the browser.

layers.offmainthreadcomposition.testing.enabled (set it to true)
browser.tabs.remote.autostart (set it to true)
dom.ipc.processcount (set it to 2 or 4, safe setting is 2, but loking at your CPU chart, 4 might be better for you)

This will enable Electrolysis, that handles the RAM in a more efficent way, also speeds up the browser. This are settings that come from Nightly builds, if you experiment problems with your extensions or crashes, reset back to defaults.

Hope this helps

i see in the subject title that it was solved.

what was the solution?

I see you try Netflix... which uses HTML5 entirely.

Along with those settings I posted earlier, you can also try:
- layers.async-pan-zoom.enabled: (set this to true)
- media.hardware-video-decoding.failed (if this is set to true, change it to false, do it everytime you update FF)

The last one disables or enables hardware acceleration for decoding H264 videos, which is the standard FF uses, even for places like openload or Youtube. Some VGAs are blacklisted (like in Chrome), so setting the ...decoding.failed to false re-enables hardware accelerations, which smoothens video playback, and should do it for Netflix as well.

If this still doesn't work, try updating your graphics card drivers to something more recent (any 2012 or above drivers may do it for FF and Hardware Acceleration).

Hope this helps.