Read: We don't want to get sued out of existence by Microsoft, Apple, and maybe Google so we're not even bothering with the US market until the whole patent thing gets reformed. You really can't blame them...
Firefox OS is really a very good concept. Most of the major websites have HTML 5 web apps that work nearly as well as their local counterparts from Android and IOS.
As I've discovered with my Chromebook, the vast majority of my needs, especially on the go, can be satisfied by web based solutions. There's no reason a phone can't have a similar setup. This situation only improves as HTML 5 based content spreads and improves.
In Latin America it's always smart to have two phones, one that's not so nice and one that is nice. That way, when you are robbed (not if but when), you can give them the cheaper phone. But you don't want to give them a phone that is too cheap, because that might piss them off. So this is a good phone for Latin America.
My aunt is very happy with her first smartphone, the ZTE Open I passed on to her after reviewing it (being one of the first people to get one).
So, yes, it's a useful device and I would easily recommend Firefox OS phones for use.
I don't know if another operating system (and ecosystem) is advantageous to consumers at this point. While Windows phone has succeeded far more in non-US markets, even IT can't garner more of a userbase and there's an entire software company (and billions of dollars) behind it. And it's not even a bad phone platform either. My guess is FF phone OS (if it deploys) will be low-end, low-budget phone, and probably won't go much beyond that.