That's because Firefox sucks, and too bad.
You must have a GeForce 8000/9000 series (or one of the ones affected). That's where Gtk+ and Nvidia's driver just doesn't work well at all when it comes to Firefox scrolling.
Nvidia doesn't care. Mozilla seems unable to fix their memory leak problems as well, as with each Firefox 3.6.x we keep hearing the same old thing: more memleak fixes.
Switch to Chrome (or chromium). It has none of these problems.