I gave up with Firefox, too many updates and too many problems with add-ons breaking, plus as said the scrolling was anything but smooth.. I now use http://www.palemoon.org/, a Fx based browser that is optimised for Windows. The smooth scrolling set up on it is very good and with the next release should be even better.
I've been using it for a while, and it seems to go slower than Firefox on my computer. I compared the two and Pale Moon seems to be going 10% slower than Firefox.

Any ideas on why this is happening?
EDIT: Also, when switching tabs, it gets a jumpy, like a framerate drop, and takes a while (in terms of a browser) to display the entire page.