If you have iOS devices, it doesn't make sense to use Firefox at all. Since the engine is still webkit. Firefox on iOS is also extremely buggy. All browsers on iOS are essentially the same but with different skins. So why not stick to Safari?
Android however is a different story altogether as Firefox can run its own engine, its own add ons etc. And it's frankly the best mobile browser.
I hope there's a new version in the next few years and it makes games platform agnostic, at least on the desktop.