It is a great time to be a web developer! I have no idea whatsoever if Firefox OS will gain enough of a stronghold to be a viable competitor in the mobile space. I have high hopes, however, that it will spur the technological advancement of HTML-based mobile apps (aka hybrid apps).
Today, hybrid apps are kind of second rate applications. Facebook (on android?) was the most prominent case, but examples abound, where the UX for an hybrid app is leagues away from a native alternative. Better foundation (frameworks/OS support) can change that, and one must applaud the unification of development stacks.