on 3.5 , flash itself would get jerky and laggy, but the browser would always be fine and responsive
in 3.6, flash still gets kinda jerky/laggy, but now the browser takes a hit too.
reproduced easily on several facebook apps on 4 different machines, this machine which has a core 2 duo running windows 7 64-bit, my i3 laptop running win7 64-bit, a p4 running XP, and an AMD Athlon II X2 running XP.
on any machine that's still running 3.5 , this does not occur.
i'd just downgrade to 3.5, but i'd have to find the installer for it, and hope i can still use my addons and keep my saved settings somehow =x
tho.. i wonder if there's a setting in about:config that may revert whatever new feature 3.6 has which may have broken flash performance