The claim that the development of Firefox OS is somehow more "open" is very suspect. Even if the source code is publicly available, and outside contributions may be accepted, Mozilla is still going to act as a gatekeeper, which inherently limits which contributions will or will not make it into Firefox OS. That doesn't strike me as being any more "open" than the development of Android or iOS is. A contribution to Firefox OS is a suggestion at best, but one that can be unilaterally disregarded by Mozilla without any sort of consequence to Mozilla. That's no different than making a suggestion to Google about Android, or to Apple about iOS.
The recent tendency of Mozilla to force totally unwanted changes (like Australis) upon Firefox users, even after very vocal objections to such changes, further makes me skeptical about how much openness there truly is when it comes to their products. Any objections, suggestions, feedback or contributions that aren't compatible with the direction that Mozilla has already decided to take appear to be ignored. "Openness" means that non-Mozilla parties can actively influence the development and future of Firefox OS. Merely being able to provide non-binding feedback or suggestions, even if in the form of code, is not really openness.
Likewise, the claim that Firefox OS "doesn't limit your freedom" is suspect, too. How can that claim be made when developers are pretty much forced into using HTML5, CSS and JavaScript to build apps? I don't consider myself to have "freedom" if, as as developer, I'm forced into using JavaScript or some half-baked "transpiler" that attempts (usually poorly) to target JavaScript.
It's also odd to claim that these apps will somehow be portable to other platforms, especially if they're using APIs that only Firefox OS currently supports. Even if there have been efforts to standardize these APIs, the fact that they aren't widely implemented by other browsers or platforms renders them as proprietary to Firefox OS, in practice, at least for some time. A standard that exists but isn't widely implemented probably shouldn't be considered a standard.
And the "HTML5 is a first-class citizen" distinction seems quite irrelevant in practice, as well. If the same apps will supposedly work on other platforms, either natively or with the help of something like Cordova, then they're just as "first-class" there as they are on Firefox OS.
So as you can see, the arguments you gave are theoretical at best, and some of them don't even appear to hold true in reality.