It had only been tested on the FireFox OS desktop simulator , how would it hold its own on a real device?
We’re not talking of flagship phones with quad-cores like a Galaxy, a Nexus 4 or an iPhone, but mainstream, single-core, sub-$100 hardware…
Well, it does! Everything is running smoothly on a Keon !
Being a packaged app, it’s only in the FireFox marketplace  for FireFox OS (ie. you need to search from a FF OS device or from the FF OS simulator), but once downloaded, it runs offline, like classic Android or iOS apps.
I also tried that other game , and it runs smoothly as well from the FF OS browser, with music and sound effects, though it isn’t playable on the Keon as the screen is too small (game was designed with the 10″ iPad in mind), but it could be playable (barely) on the larger Peak , and the announced FireFox tablet  should be no problem.
Is FireFox OS ready just yet? Well, the shell could still use some work in the smoothness and style departments, but for apps, things do looks quite stable and fast.