Engadget posted the images, confirming that Marketplace will play host to e-books, business applications, games and other software.
Several free apps were outed in the screenshots, including World Wars, Nook, Lanyrd Mobile, Solitaire and Galactians 2.
Mozilla announced plans to enter the mobile operating system market back in February. Sprint and ZTE are on board as launch partners, with the first phones running the OS expected to appear in 2013.
Firefox OS had been built on top of a version of Linux, but runs applications in a mobile version of Firefox.
The first Firefox OS devices will debut in Brazil before rolling out into other territories.