As I understand it, Gecko has significantly lighter resource demands compared to Dalvik. Andy Rubin himself, speaking on FirefoxOS, has said that there are simply places where Android cannot go.

Cite: "There are places where Android can’t go," [Rubin] said, referring to memory and other hardware requirements. Firefox can help reach those. "For certain markets, it makes sense." [http://allthingsd.com/20130226/googles-andy-rubin-on-firefox...]

Because that's what Mozilla keeps telling me.