Furthermore, why do you say emerging markets don't need it? The other OSes do not work on low-cost handsets any more. Targeting that niche is a good idea as it gives the OS a chance to gain relevancy with users who won't be as picky as HN readers.
That isn't how reality works, though, obviously. Out of necessity, most users must place software usability and capability above ideology. Software like Firefox OS and Persona will never see serious adoption when they can't compete with long-established and more functional offerings at the most basic levels, even if these competitors may not be as "open".
It's an approach that doesn't work for Mozilla's offerings that are already well-established, either. Firefox has been hemorrhaging users ever since its developers stopped focusing on truly improving the user experience, and instead focused solely on copying the worst aspects of Chrome (the UI, for example), while neglecting to address the performance and resource usage problems that have long plagued Firefox.
Users need software that works. If that means using software that's "less open" or "closed", they'll do it without a second thought. Mozilla just happens to often be on the losing side of this reality these days. While "openness" can be beneficial, it needs to be in conjunction with software that's at least comparably good to its "closed" competitors. Firefox OS, Persona and Firefox are good examples of where this isn't the case, and how they're either seeing limited to no adoption, or how they're losing existing users.
Also, Mozilla are playing a smart game with FF OS. They are not selling to the high-end or even mid-end markets. They are selling it as a modern OS with much lower requirements than Android for low-end budget phones. This is a niche nobody else targets any more. It forces them to improve performance and memory usage, too.
On Android, it's also my browser of choice - fast and smooth, and it gives me the option to block third-party cookies (been surfing the web w/o them for 15 years - but no, mobile Chrome cannot allow that; we need more adver-tracking!).
While I'm a very big fan of native, optimized code (that is: C), I think FF OS has an advantage concerning memory use, because the browser and the rest of the OS share the runtime, unlike an Android phone. It also makes it easier for low-budget developers to write apps, because they don't need (a machine that can handle) an SDK, just a browser.
Whereas with Android and iOS you have far less language choice.
It's as if Oracle/Microsoft was saying "oh, you can use Java/C#" while simultaneously hypocritically making every effort to make bytecode/msil a mainstream stand-alone programming language of it's own.
And again, you have to demonstrate that mobile platforms have good language choice in comparison to the web. So please talk about all of the language choice available on iOS and Android.