This is not correct. Both the old Firefox for Android(codename Fennec) and the new one(codename Fenix) use Gecko, Mozilla's rendering engine.
What has changed is that Fenix uses Gecko via https://mozilla.github.io/geckoview/
What has changed is that Fenix uses Gecko via https://mozilla.github.io/geckoview/
Since I have had issues in my app with Chrome/Android WebView, I will start experimenting with GeckoView. I didn't know it existed as a component. Thank you so very much!