Something important to note compared to android is that android phones in this range are stuck with gingerbread and no official play store.
Android can increasingly be found on a lot of cheap, low-cost smartphones too - and there's a large established app marketplace. Firefox OS has a couple of potential advantages though. If the Firefox team can deliver on fast native performance for HTML apps, then development for the OS will be magnitudes easier (potentially) than developing for rival OSes.
User privacy is another feature where Firefox OS could easily outshine its rivals. I'd love to see a Firefox OS laptop that goes head to head with ChromeOS for example (Google's OS tracks everything you do - even printing to your desktop printer).
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.
[1] It's not a normal place http://www.walmart.com/ip/Jimmy-Dean-Croissant-Egg-with-Red-...
[1] http://www.amazon.com/Nokia-Lumia-520-GoPhone-AT/dp/B00E4504...