Last Update November 3, 2020: We improved the overall article.
What is Firefox OS
Firefox OS is a Linux kernel-based open-source operating system for smartphones and tablet and is set to be used on smart TVs.
Firefox OS is designed to provide a complete community-based alternative system for mobile devices, using open standards and approaches such as HTML5 applications, JavaScript, a robust privilege model, open web APIs to communicate directly with cellphone hardware,and application marketplace. As such, it competes with commercially developed operating systems such as Apple’s iOS, Google’s Android, Microsoft’s Windows Phone as well as other community-based open source systems such as Ubuntu Touch.
about:app-manager) Or 
Install Here : https://ftp.mozilla.org/pub/mozilla.org/labs/fxos-simulator/
After Install this Addons In App Manager’s bottom toolbar, click on Start Simulator, then click on the name of the installed simulator, which should appear there.
Install ADB helper
The process uses the Android Debug Bridge (ADB) to handle the device-computer connection and communication.
Let Firefox handle ADB (recommended). Install the ADB Helper add-on, which makes the process easier. With this installed, there’s no need to install the ADB, and no need to type the adb forward command: everything is handled by the add-on.
https://ftp.mozilla.org/pub/mozilla.org/labs/fxos-simulator/
Connecting your device to the App Manager
With all your configuration done, it’s now time to plug your device into your computer and start the App Manager:
At the bottom of the App Manager tab, you will see a connection status bar (see screenshot below). You should be able to connect your device by clicking the “Connect to localhost:6000” button.
Now that everything is working, let’s review the functionality available inside the App Manager, starting with the Apps panel
