I noticed in the post The Truth About the Spinning Dancer, the animated GIF of the dancer spins significantly slower in Safari 3.1.1 vs. Firefox 2.0.14 running Mac OSX 10.5.2. This is with a fresh launch of each program and only the page showing. At first I thought it was the computer that I use at work, the original MacPro 2.0 Dual G5, but it’s the same on my home computer a MacPro Intel Quad Core.
CPU usage is actually about 30% more for Safari vs. Firefox while the animation seems to be about six times faster for Firefox.
Scrolling the image off of the screen reduced CPU usage by 50% in Firefox. While in Safari, scrolling the image off screen reduced CPU usage to nearly zero.