"They've always said they are aiming to target cheaper less powerful phones"
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).