Some stats from the benchmark running Windows 8 and Ubuntu 13.04 on Asus Zenbook UX21A (Intel HD Graphics 4000, Intel Core i7 3517U, 1920x1080):

    OS            Browser      Avg. FPS    Min FPS
    Win8          Fx 20        26          16
    Win8          Fx Nightly   46          34
    Ubuntu 13.04  Fx 20        17          16

For me those numbers tell that if ams.js does not catch on in other browsers, browse based games just will not fly. Also, graphics drivers on Linux still suck and that's why I still need to double boot.