I probably read like a broken record at this point:
Despite Mozilla dropping development on it, FirefoxOS is still not "dead". Open source things only die when no one has interest in them any more. AFAIK FirefoxOS is still the most open mobile operating system. If some other company decides to take up where mozilla left off, fork FirefoxOS, and do stuff with it, they can make a run at it (this is extremely unlikely).
The platform itself is pretty amazing already - It does all the actual "phone" stuff just great -- making calls, texting (minus a snafu with SMS group messaging on the version I'm on). The apps are web apps. The phone is almost literally one huge browser. There isn't that much more to add! Yeah it'd be nice if it had more "native" app support, but honestly, any site with a mobile website (or responsive design) has "native" app support. If you've used version 2.x, you're probably pretty satisfied with what your phone does now anyway. I know I am.
FirefoxOS will still be useful as long as people keep making apps for it (and again, apps are basically webpages, so even if it's not distributed on the marketplace, people can still webpages to homescreen), and it will be useful as long as people want to use it. It is already viable open source mobile OS. I am grateful to mozilla for taking the time to make it.
I'm excited to see what they do in connected devices, because despite the moves mozilla has made recently (hello, pocket, addon signing requirement), I still would much rather have them build software that was running on my toaster than anyone else.
[EDIT] - Also, check out the comments on the site. They're great.