It seems like recently the number of firefox browsers has increase by quite a bit, so we should maybe add some of them to devicedetector:

Firefox Lite/ Firefox Rocket ✔️

https://play.google.com/store/apps/details?id=org.mozilla.rocket
https://github.com/mozilla-tw/FirefoxLite

It seems to be quite similar to Firefox Klar/Focus, but focus more on saving data by not loading ads, images, etc. and working on very low resource devices.
It's based on webkit/webviews and developed by Mozilla Asia.
The user agent seems to be quite helpful:
Mozilla/5.0 (Linux; Android 9; ONEPLUS A3003 Build/PQ3A.190505.002; rv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Rocket/1.6.2(12322) Chrome/74.0.3729.157 Mobile Safari/537.36

Firefox Preview / Fenix

https://play.google.com/store/apps/details?id=org.mozilla.fenix

This is the main proof of concept that should soon replace Firefox on Android and become the main browser. It's based on Gecko Android Components with try to be similar to webviews, but with Mozillas browser engine.

Unfortunatly the user agent I get on my phone is not all that helpful:
Mozilla/5.0 (Android 9; Mobile; rv:68.0) Gecko/68.0 Firefox/68.0

If someone has some ideas on how to differentiate these browsers, I'm all for it.