After disabling a couple old add-ons that were blocking Firefox's multiprocess architecture (Electrolysis), Firefox now feels as fast as Chrome for me.
I find it's easiest to restart with add-ons disabled (under Help) and confirm that add-ons are the problem first.
To narrow it down to the add-on(s) causing the problem, disabling all of them under about:addons then re-enabling one at a time is a good approach, although can be time-consuming if you have a lot of add-ons installed.
Also - about:performance might give you some clues.
