[glogla]

There are different kinds of freedom and openness.

FxOS is made by Mozilla, which is very different in both goals and culture from Google, Microsoft and Apple. You don't need special account with OS vendor to unlock full functionality of the phone like with Android and WP, or to even use it at all as with iOS. You don't need special license (that actually costs money, IIRC) to load you own software to your own device like with iOS or WP. You can develop the software for FxOS on any operating system, unlike with iOS and WP.

This is different kind of openness than "can I use C", but to me it is more important. Personally I hate Javascript with strength of thousand suns. But FxOS seems to be worth the price.