There are a lot more people who can write a web app than a mobile app, by a factor of 10 at least. Give them a platform where it's trivial to make an app (easier than, say, Cordova/PhoneGap), and make it free, and you could easily disrupt the mobile market to a small degree.
If FirefoxOS is a success the next challenge will come from the fact that iOS and Android could easily have HTML5 apps too with a native WebviewUI 'player' app, and if FirefoxOS gets any traction their respective developers will add that functionality quickly. Hopefully, for consumers and developers alike, they'd all have compatible APIs so mobile apps would be truly cross-platform, but that's not very likely.
At this point "a small degree" won't amount to anything. Look at how much money Microsoft have had to burn through to get the few percentage points they've managed to amass. And they still can't get developers onto their ecosystem despite the bribes they've thrown out.
It's go big or go home time and Mozilla simply don't have the resources to do it.
If FirefoxOS is a success the next challenge will come from the fact that iOS and Android could easily have HTML5 apps too with a native WebviewUI 'player' app, and if FirefoxOS gets any traction their respective developers will add that functionality quickly. Hopefully, for consumers and developers alike, they'd all have compatible APIs so mobile apps would be truly cross-platform, but that's not very likely.