I have found FF7 to be incredibly light compared to previous versions. I had all but switched to Chrome until the latest update came out. Most people that I know of who have horrible speed problems associated with Firefox have numerous unnecessary add-ons running all the time.

If you are experiencing speed and CPU consumption issues try doing a spring cleaning of your add-ons. It has always been the best way to fix the problem whenever I have people complain to me with the same problem you are having.