well the other option is to build web applications that need these technologies, kind of like PDF.js and jsmad have done, but for some of the other missing technologies that you have listed and see what is missing. You don't have to build an OS to see what is missing - that's my point.
I do not understand your point. How is a pure web application going to implement device APIs for Camera, Mic, GyroCompass, Accelerometer, USB, NFC, the list goes on?
These must be provided by the OS. But various OSes do these differently, some only for native apps. We propose to make every one of these a web-app-facing API, and standardize or use extant standards -- and include security up front and all along, not "add it later".
No one has done this, and saying that you don't have to build an OS does not address how web apps might come to have such device APIs.
Of course the web applications themselves wouldn't implement these features, but they would act as test cases for the APIs that get added to the browser (e.g. Firefox). As you build these test case apps, you'd see what features are missing and implement them in the browser API, standardizing things along the way. If your point is to improve the web APIs available and find out what is needed, then build the features in the browser and build great applications on top of these new APIs that work on all platforms rather than just being tied to the Mozilla OS (at least for a while). If you truly want to make these features into standards you'll be adding them to Firefox later anyway, right?
The problem is that Firefox can't implement these APIs without OS support, and OSes don't have the same kind and number of such APIs. We are working on better cross-mobile-OS Firefox, but it cannot be our only bet.
What's more, Firefox is locked out of many mobile OSes (I do not mean iOS in particular), but the commoditizing hardware can support a fully open OS and web-based "home" and open web apps environment.
The increasing vertical lock-in and tying among all OS vendors (Android included) and the at best half-open- / delayed-open-source status (ChromeOS excluded, bless it -- but it won't support other browsers than Chrome) are problems for Firefox.