I believe that a cloud OS for phones would be the cause of that phone's demise. You don't get 3G data wherever you have phone signal, and if you are out of your home network's range, you don't get any signal. Waiting for a phone to load all the OS components for a few minutes on EDGE is a dismal prospect, as is having a useless brick in case you're out of your network's coverage.
This would not work even in the rest of the world, because people that travel will need to pay for connectivity (roaming, etc) just to be able to boot their phones. With the current (and likely near-future) business model of wireless providers everywhere, this design would be stillborn.
I think Gruener missed this point. If anything, a phone still needs to have an OS that resides entirely on the phone, not in the cloud.
OTOH, if Mozilla comes out with an OS that is able to unify/run applications from different markets, and that OS is compatible with a wide variety of hardware, that would be something I would install on my phone in a blink of an eye. So, yes, we need such an OS, again despite what the author thinks. Granted, the AppStore will still be off-limits, just because it requires registering via another program (iTunes) that will detect the hardware and deny access, but with compatibility for alternative markets, that should not be missed too much.
I use FF daily and I believe they are capable of pulling something like this. I also have a few old(er) smartphones laying around that I could use for testing such an OS. I also know for a fact that there is a huge community of developers and coders out there waiting to jump on every opportunity, just like they did with Android, iOS and others.