Mozilla is not going "against" others for the sake of it. We build what we feel is better for the web and its users, and we do it where we think we stand a chance of being successful at a large scale.

As for proposing an alternative for ChromeOS, I'm mentoring a GSoC project to turn firefox OS into a Desktop Environment (see https://wiki.mozilla.org/Community:SummerOfCode14#Firefox_OS...) which could be a first step in this direction.