The scrolling issue on Firefox is related to having smooth scrolling enabled. Turn it off and it scrolls fine again. I'm assuming its the same for IE.
The CPU usage is directly related to smooth-scrolling. When its enable on Firefox, Opera or IE the usage shoots way up while scrolling. With it off its only a few percent. I don't plan off turning it off for one website. I'll just make Chrome (which doesn't have it as a feature) my Beth forums browser for now.
I use the YASM (Yet Another Smooth Scrolling) addon in Firefox since vanilla FF scrolling is much too bouncy for my liking.
With YASM off, my CPU usage still can hit 50% scrolling up and down the Oblivion modding forum. In contrast, doing the same activity on [H]ardForum, my CPU usage hits <8%.
Like you, there is no way I'm turning off smooth scrolling (YASM) for a single site, and since Chrome's extensions are still basic, I'll have to keep using FF since I am not a fan of having two browsers open simultaneously all day (I browse the modding forum throughout the day and always have it open in a tab if I am not actively browsing the forum). Once Chrome becomes a bit better in the extensions department, then I plan to switch from FF to Chrome and can remove this annoyance from my list that this site's redesign has brought.