Windows: I still couldn’t find any resolution other than disabling acceleration and have quicktime use safe mode.
Mac: I ran a check and fix file permissions just for the heck of it. It did find some problems. After everything is fixed, Firefox and Quicktime seem to behave better now. Quicktime videos running in Firefox (like the iLife 08 guided tour) run okay now, although they still seem to be sluggish and hog the CPU.