Firefox OS emulator

The guys at Mozilla released a plugin for Firefox so you can try out Firefox OS emulated. Looks like it's an embedded JavaScript emulator, but I warn you: it runs quite fast and smoothly, definitely much faster and smoother than iOS/Android emulators. This is probably due to the fact that Firefox OS is heavily based on JavaScript. So what do you think about this new mobile OS? Do you agree with the use of HTML5 for building the applications? Do we need yet another OS? Should Mozilla better focus on their browser?