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/
Thank you so much for that link. I think I got mislead by heise.de into thinking that this issue with Chrome WebView existed.
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!
Neither version of Firefox for Android uses a Chrome webview. The old one is built on Gecko directly, while the new one uses GeckoView.
This Firefox Nightly one uses the Gecko rendering engine, while the standard one uses Android's (or Google's?) Chrome Webview. Has this changed?