Firefox OS mobile devices are notoriously crap, really weak obsolete hardware, made to fit in a very small $ budget. If so many people are complaining about poor performance on Android and iOS, they will have a nightmare on Firefox phones.
The browser for desktop however is great.
That's old news, Mozilla evangelist recommended me to update Firefox 1.3 introduces WebGL support, I decided to update my keon (Firefox OS Developer device) and I tried to run my medium game that couldn't run my iPod touch 4, then on Fireox OS, it runs like... wow awesome, it blew my mind how it runs with smooth fps.