I think OSX uses a taxing algorithm on the rMBP to stretch and render images, animations, videos and fonts using 4x the amount of pixels, this extra algorithm seems to be the culprit in my opinion because it taxes the GPU, Safari has been optimized to scroll smoothly using said algorithm while Chrome and Firefox are not optimized for such task, so they both lag horribly when scrolling, if you want to use a navigator other than Safari, I recommend using Opera, this navigator seems to be almost as fast as Safari when scrolling.