I Think WP and FirefoxOS are the two big players in the low-end smartphone market, and the victory of one of those depends on that segment of phones.

WindowsPhone8.1 already allow developers to use html5/js to create native apps (using winJS or any other framework) and the smartphone that sold most is the lumia520 -> the most low cost of all the wp gamma.

I hope FirefoxOS will succeed, but considering that MS is trowing literally Billions to keep wp alive, I doubt it will be easy.