From my observations Firefox is considerably faster than IE 8 with regards to displaying pages (note, I do have NoScript running in Firefox so this most definitely contributes to this), however the CPU usage of IE is almost always lower. Firefox has a tendency to spike when only viewing HTML pages while IE typically never does this. If a page is using Flash, all bets are off.
Of course, if you load up Firefox with add-ons there is going to be some impact, but I only use 2 that impact page content: NoScript and Flashblock. So I can only conclude that Firefox takes a “greedy” approach to get what it needs done as fast a possible where IE is more “considerate” by using less CPU for a longer period of time. Of course, this is not a scientific or conclusive study it is interesting to see the two different approaches. Whether this behavior is by design or coincidence, who knows…
Granted, CPU usage is not nearly as critical as it used to be as most all systems today are multi-core, I still believe this is an important indicator to keep tabs on.
What observations have you noticed on the browser(s) you use with respect to CPU consumed? Does this even matter to you?