Mozilla has released a Firefox plugin Firefox OS Simulator that helps you to create and test your own Firefox OS apps. You can download Firefox OS Simulator plugin form Firefox extensions repository just like any other plugin. Once installed, you can run this application from Tools > Web Developer > Firefox OS Simulator menu.
Firefox OS Simulator allows you to move from screen to screen, open any app, install apps from the Firefox Marketplace or any other domain. Check out the above video that showing Firefox OS Simulator in action. So grab Firefox OS Simulator now and start developing!.