> In fact, they're trying to encourage web apps that run on every device over native apps that only run on one platform.
Not quite. They are encouraging apps that are based on HTML and Javascript, but as far as I can tell Firefox OS includes plenty of APIs that are not available on other platforms, and there are no plans to bring them to other platforms [1].
Yes this isn't necessarily the fault of Mozilla, they can't force Google/Apple/Microsoft/Opera/Nokia etc to add features to their browsers [2], but that of course is one of the problems of using the 'open' web as a platform for everything. Either you remain standards compliant with very limited features dictated by the glacial/gridlocked standards process, or you extend the standards even without the support of the other big players, resulting in apps that only work on one OS (although are perhaps easier to port, depending on exactly what is missing).
[1] https://wiki.mozilla.org/WebAPI I see plenty of grey "not currently planned for this platform" items in this list
[2] other than to release their own browser on those platforms, which brings us full circle… the most important thing is open platforms where anybody can write and distribute native software targeting them