Firefox OS is a seperate mobile operating system (like Android), Firefox for Android is just a browser for android.

Firefox OS uses the Gecko engine for its "Firefox" like browser.

For Android phones you can find Firefox for Android on the Google Play store. https://play.google.com/store/apps/details?id=org.mozilla.firefox

As Waka says Firefox OS would require re-flashing your phone to change the operating system. This is a dangerous procedure and can result in your phone becoming non-functional. Assuming that this device has a Firefox OS port in the first place.