Firefox OS devices are slowly becoming available, and it's as difficult to test your app on a device as it used to be difficult to test your app on B2G Desktop (until the Simulator made it easy), because you have to acquire tools like adb, configure them, and then issue commands from a command line to do so.
We should make it just as easy to test your app on a device as it is to test it in the Simulator. Specifically, once you have added an app to the Simulator and connected a device to your computer via USB, the Simulator should detect the device and give you a simple visual interface for installing/updating/running your app on the device. It should also let you debug your app remotely and otherwise interact with it in all the same ways you can interact with it in the Simulator.
@fabricedesre has written some code in https://github.com/fabricedesre/b2gremote that communicates with a device using adb. We might be able to use his code to implement this feature.
This was implemented in recent commits. There's still more to do, but we'll track that work in new issues.