So it's an OS that requires apps to be written in JavaScript (an interpreted language that was designed by Netscape for web scripting and nonprofessional programmers) and HTML (a markup language designed to display static documents in the browser). None of these technologies were designed to help developers create software applications, so why is this magical?
Let's look at few possible reasons:
1. JavaScript/HTML/CSS are better tools for developing software than anything else out there. No.
2. It's open source. So is WebOS, Android.
3. It's more efficient - same software can run on cheaper hardware. No, Java & Objective C are more processor-efficient. If Firefox OS runs on $100 hardware, than iOS can run on $50. The fact that nobody is making an ulta-cheap smartphone is because there's little demand for one (it's like making a shitty Porsche)
4. Developers can package existing web applications as native mobile applications. There's already a thing called PhoneGap for that.
5. More teenagers know JavaScript than any other language
It seems like #5 is the only valid reason. A lot of kids don't know Objective C or Java, so they must love Firefox OS if they just know the web stack. But sometimes you've got to ask yourself - why rewrite an OS in a new cool language? Just because you can? Look how far Apple has come with Obj-C, why constantly replace the hammer? Having said that, the force of teenagers is not to be underestimated.