The Firefox OS Simulator is a version of the higher layers of Firefox OS that simulates a Firefox OS device, but runs on the desktop. This means that in many cases, you don’t need a real device to test and debug your app. It runs in a window the same size as a Firefox OS device, includes the Firefox OS user interface and built-in apps, and simulates many of the Firefox OS device APIs.
The Simulator is packaged and distributed as a Firefox add-on. Once you’ve downloaded it and installed it in Firefox, you can run it, push apps to it, and attach the developer tools to it, using the App Manager.
Installation – Multiple versions are available, and you are advised to install them all, for maximum flexibility)