So is Firefox OS just Android fork? Because if not then this isn't as much "Firefox OS on Android" as it is "Firefox OS like home screen for Android"
[1]: https://mxr.mozilla.org/mozilla-central/source/widget/gonk/l... is pretty much all taken from Android.
Parent is both technically incorrect as well as semantically incorrect. FirefoxOS uses a large amount of Google-written Android code.
So in this case you are running Firefox OS on Firefox on Android. That is obviously not ideal, but fine for testing.