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.
