I'm not so sure they are solving the same problem they were solving before. "The web" doesn't have to be a browser anymore. It can be any app. For example, the Facebook app is a native app that makes a bunch of http api calls to provide the FB experience. Previously it was basically a web app with a native wrapper. In both cases you don't need Firefox to use Facebook.
I think native apps calling http services is a step fowrard in a lot of ways because if Firefox screws up or doesn't implement a feature the right way or whatever, your app isn't beholden to Firefox to fix their stuff in a future release.
Also, when there was basically just IE on Windows Firefox was definitely a reason for Firefox to exist. We have good browsers that seem to put a lot of effort into supporting mobile and HTML5. They're not perfect, but it's not even the same as IE6 back in the Win XP era.
Choice is good, but I don't think FirefoxOS is fixing anything.