The Firefox Marketplace is going to be Mozilla's app store. It's already Mozilla's app store, but it won't be all that useful or used until Firefox OS comes out. That's not necessarily a bad thing; it means Mozilla has time to work on it.
In fact, the Marketplace has gotten a redesign, a complete revamp, even if the big Firefox OS launch is months from now.
The new design is cleaner and sleeker looking, which is a good thing, as Firefox OS needs to look as professional as possible when it comes out.
The layout is optimized for the touch, but it works great on a desktop browser too. The apps are meant for Firefox OS, but they run in the desktop Firefox as well, a benefit of using web technologies.
Mozilla planned this all along, as it wants the app ecosystem to be cross-platform. It may not be a bad idea either.
Google is doubling down on desktop web apps as well, with the next-generation packaged apps for Chrome.
