Cool beans. It's a really cool idea, because every smartphone OS needs a web browser anyway, so extending the web browser to the rest of the phone instead of having two separate widget/display systems seems to be really doing nice to performance. I like how they are turning all their JS APIs into W3C standards. This could also advance web technology too in general, by making it easier to make websites that interact more natively with your phone's feature set.
My concern would be of course, apps. That's the struggle any up and coming OS needs to to attract the level of developers Android and iOS have. I don't think apps are as much of a problem on the phone as they are on the desktop, but people will expect the standard set of apps, and they'll get frustrated if something they depend on isn't available.
The good thing though is if you write apps for Firefox OS, they would be at least partially portable to Android and iOS, because both support web apps as native app targets. So who knows.