A 256MB RAM, in volume, would cost about $2.5-5 in volume. If that's the only difference between Android with 512MB(which is very usable) and FirfoxOS device, that doesn't sound like a lot.

Also opera/opera-mini offers much better performance on mobile than mobile firefox,and opera mini is very light on ram(maybe 30MB-40MB). So while i haven't tried, i wouldn't be surprised to learn that running opera mini on a 256MB android offers better performance than FirefoxOS.