Hey PommeDeTerre,
Allow me to give a different perspective here. While in many other mobile operating systems you can build apps with HTML (saying HTML from now on but meaning HTML/CSS/JS, ok?), the API the vendors make available don't cover the same features as what is native on that platform. We can say that Firefox OS is important because of the following things:
MAKING THE WEB A FIRST CLASS CITIZEN
Makes the web a first class citizen. Its not like "you can build apps with web technology", its more like you don't need native because all you need is available thru the web apis. While in other platforms, there is a barrier on what can be accomplished with HTML alone, on Firefox OS HTML is the way. Let me give you a silly but practical example:
Suppose you have an intranet on your small company built by IT in PHP or something. This intranet has an address book for everyone in the company. Using the Web API, you can detect the contacts API and offer the user the ability to import contacts from the web directly to the phones contact book. This is done in a secure way, with permissions granted by the user, its not like "hey, open this page and flood the contact database". The security model is good. This is a silly example but it is useful for many people. Its the tiny APIs that make your life easier and your experience better.
GIVING YOU THE FREEDOM OF THE WEB
In platforms such as iOS, your only venue to distribute your apps is the iTunes App Store. All money passes thru Apple. On Android, you can have third party stores but the default setting is to allow apps from Google Play only (unless you're a kindle or nook). Most of the other platforms are about controlling the ecosystem to make money for the vendor.
On Firefox OS you have the Firefox Marketplace that is a mixture of Chrome Webstore and Google Play. It allows distribution of apps for desktop, android and firefox os (all using gecko runtime). You are not tied to Firefox Marketplace though. You have an install API that you can use on your app or your site to install apps on the users phone (Again, it asks permissions, security is good). You can even fork the Firefox Marketplace code and build your own.
Talking about payment, you have the usual credit card charging on the marketplace but you also have carrier billing (if the carrier implement it) allowing users to buy apps and have it charged to their prepaid cards or monthly bill. Remember that in many markets, people don't have credit cards. If you're not happy with both solutions you can roll your own payment solution using the payments API and receipt protocol. You are free to distribute and monetize your app as you see fit, just like on the web.
ITS NOT A REPLACEMENT FOR YOUR IPHONE OR GALAXY S3
If you have an Android device, you can just install Firefox Mobile and be happy since we'll eventually have feature parity between Firefox Mobile and Firefox OS. These Firefox OS phones are being created for emergent markets. People from U.S. and richer parts of Europe don't really realize how things are in south america and asia. An Samsung Galaxy S3 will cost USD 1000 in Rio right now while the minimum wage is about USD 350. Buying such expensive phones require a plan and saving for a long time. 78% of the Brazilian market is on feature phones.
By offering an inexpensive handset that has a full smartphone experience with a price that is affordable is key in these markets. It is about bringing the next million people online on the mobile web for the first time. Firefox OS is here to be the first smartphone for people that are still on Symbian or S40. In some markets you can buy a cheap android with decent performance, in others you can't.
Firefox OS offers the best cost-benefit ratio once you think about how much performance you have against your hardware specs. If you pick the same low-end hardware with Android or Firefox OS, Android will have a worse experience, not because Android is broken in any way, Android is great but because there are more things going on on Android than on Firefox OS. Firefox OS is just a tiny hal, gecko and gaia, while Android has a VM and lots of other things going on.
As I said before, this is not about fighting Android, its about offering a new choice for those that can't afford a good Android or iPhone. Here in Brazil, we have some very cheap Android devices since we have Samsung, LG and others building them locally. Most of these devices are pretty crap to use. The same hardware with Firefox OS would be a much better experience.
ITS ABOUT THE WEB API
The objective here is not to sell phones or be dominant. Firefox OS is about preventing vendor lock-in. Right now, each platform is an island. iOS has Objective-C/Cocoa Touch, Android has Java, Windows Phone has something that I can't recall. If you want to build cross platform apps, you're in for a world of hurt. You can use phonegap but this is not something offered by the platform vendor, this something that was built to bridge the web and native.
The Web API is the next step in this direction. Each of the APIs in the collection known as Web API is being standardized with the W3C and other interested parties. The idea is that other vendors adopt these APIs and that open web apps work anywhere equally. Firefox OS is not about an exclusive experience, it is about openness and freedom. Even if Firefox OS fail but the Web API succeeds, it will be a huge victory for the open web.
ITS ALSO ABOUT BEING CREATIVE
Right now there is a steep learning curve regarding developing native apps for most platforms. Objective-C and Java are easy languages for CS people but for the layman they are pretty hard. It requires a lot of effort to be good with them.
Why is the web a success? If the web required such effort, we'd still be using gopher and news. One of the reasons the web was a success was that it required less effort to produce something useful. I am not saying good, I am saying useful. Anyone could host a server, anyone with a text editor could build pages. Firefox OS is about bringing this to mobile. It requires less effort and your open web apps can be deployed on other platforms such as iOS and Android (with phonegap or other runtimes). Yes, these days it requires a lot of effort to build good web apps but not much to build useful ones. Firefox OS has the potential and openness to bring all web developers to mobile with ease. Open Web Apps are king of local content. Niche apps that are only useful for those in a neighborhood or profession are a web editor away from being born and available. The web is king for local content and Firefox OS is nothing but an mobile extension of the web.
WHY I THINK FIREFOX OS WILL SUCCEED?
As I said above, Firefox OS is nothing but the web and the web can't fail because it already succeeded. Right now the only technology that bridges smart tvs, video games, smartphones, desktop and laptop computers together is HTML/CSS/JS.
Remember how much progress we had in JS runtimes the last year, try to imagine where we're going to be next year. Javascript is already reaching 1.5x native speed in some cases, two years from now, this won't matter because it will as fast as it needs to be. One could argue that for most mobile app needs, it is already fast enough.
Apps built for Firefox OS can be deployed as web pages and made available on a server. They can be sold on Chrome webstore and Firefox Marketplace. They can be wrapped with phonegap and deployed to iOS, Android, Windows Phone, BB10, Bada, Tizen and webOS. They can be wrapped in other runtimes and be used on Smart TVs and video games. The web is the king of cross platform, of vendor neutrality and without lock-ins.
I think that the future belong to HTML based solutions and all platforms that embrace it.
PS: I am a Mozilla Rep and a Firefox OS Launch Team member in Brazil.