Usually, if you want to try a new operating system or even upgrade your existing one, you’ll need to mess about with boot loaders and watch as your phone suddenly becomes unsupported.
How about trying a whole new OS by just downloading an APK? You’ll need to accept and run an APK from outside of the Google Play store, however it will let you take a peek at the Firefox OS and switch back to Android when you wish. Whilst the Firefox operating system doesn’t run on a huge proportion of smartphones, it’s still good to see how the thing works.
Things are a little slow, and it crashed on me twice, but it will help you to build an app for an operating system that uses HTML. Once installed it’ll take over as your default home screen but you can still run Android apps alongside the Firefox OS ones. To uninstall you’ll need to switch back to your old home screen and then remove the Firefox home screen.
If you want to give it a spin, head over to the Firefox OS website. If you can do HTML and CSS then you can start to develop for the Firefox OS.