B2GDroid (Firefox OS) App
Try the Firefox OS without uninstalling Android
In this case, the app installs a personalized launcher that lets you use Gaia, the menu system used in the Firefox OS, along with some of the associated apps like its own messaging service, contact list and, obviously, the integrated Mozilla browser. In addition, it's possible to enter the Mozilla marketplace and download apps in HTML5 to your device. To run them, you'll have to press the Home button and select the Firefox OS as the default launcher.
Even though it's possible to install the Firefox OS in your device from scratch through complicated processes, this is the best way to try it out and get an idea of how it works without making any huge changes to your device.