I noticed this around version 49. Through process of elimination I found out it was caused by two plugins I had: Privacy Badger and Adblock Plus.
Switched to uBlock origin and removed the other two, performance is normal again.
Switched to uBlock origin and removed the other two, performance is normal again.
On linux I also saw a lot of benefit to making sure hardware acceleration was force enabled (it's disabled on the platform by default for some reason; I also had to start optirun-ing it on my laptop). Firefox has literally never been faster or behaved better since making those two changes.