The one major benchmark I use for browser speed is how fast Tagpro runs, and it's definitely gotten much slower on Firefox recently, to the extent that I need to generally play it on Chrome, even though I normally much prefer Firefox to Chrome.
Edit: Just to be clear, when I say much better, I don't mean much better than chrome, just better than it performs now. When I drop RAF and just run the rendering loop as fast as possible, they get roughly the same FPS on my cheapo laptop.