I've seen this too. However, they seem to have made improvements in Firefox 15. Previously, Firefox would take around 3% CPU with just the start page open; visiting a page with an animated GIF would push it to 12%; and, most troubingly, CPU usage would stay around that level even after the page was closed. Now, the same page still takes around 11%, but Firefox stays at 0% with just the start page open, and it goes back to 0% once you close other pages.
