The benefits you list for Firefox OS may not apply to iOS, but they do apply to Android, too. I can install apps on my Android phone without using any app store, for instance. I can freely write my own Android apps, and distribute them myself, without having to pay anyone or get permission to do so. I don't have to upgrade to a newer version of Android if I don't want to.
If I can already get all of that with Android, and so much more (such as decent performance, the ability to develop apps in languages other than JavaScript, access to a huge number of existing applications, and so on), what is it that should be compelling me to use Firefox OS instead?
Furthermore, various incidents since the release of Firefox 4 have also shaken my trust in Mozilla. Time and time again, we've seen decisions made with very limited community input, and apparently little regard to the impact that they'll have. I'm talking about how the approach to releases was changed in a way that broke extensions time and time again, for a long time. I'm talking about how the status bar has been removed, and the menu bar was hidden by default. I'm talking about how the protocol isn't shown in the URL bar. I'm talking about the latest incident from today, where the preferences dialog option for disabling JavaScript will apparently be removed in Firefox 23.
Incidents like those make me very skeptical of the "openness" claims we hear time and time again with respect to Firefox OS.