Same here. Part of the problem turned out to be related to youtube: https://www.reddit.com/r/firefox/comments/41zh1y/sick_and_ti... (Don't be misled, it affected MUCH more than youtube - I also had very little subscription, 5 or so).

Nevertheless, Firefox is part of the problem. And performance is still sub-par since I applied the fix (although not quite as bad).

Gotta love the people that answer "works for me".