That looks not suspicious at all. That the CPU-Usage of FF varied is normal but I see Window-Update, that could cause problems even without high CPU-Usage.
I've run out of ideas, what you could else try:
No internet access. Turn your Internet access off
Wait until all windows updates are installed.
You could try with a non browser 2D scrolling game if you have the same stuttering (I recommend Hydorah http://www.locomalito.com/hydorah.php
(80MB slow server)) and if Videos on YT with the html5-player stutter.
Boot your PC complete without all extern plugin Devices (USB) (except Mouse and Keyboard) even Lan-Cable or WLAN-Adapter. If you have build in WLAN turn it off.
And last try Windows in Safe Mode that should all turn off, even drivers.