Interesting, so for the apps that I've used that open a webview (which specifically says in the menu box "Powered by Firefox", with an option to "Open in Firefox", which opens in the full browser), they've added the library specifically in the app? To be clear, I don't have a rooted phone.

Not trying to be difficult, but I'm seeing this specifically in the Fastmail Android app, and I don't remember ever setting an app-specific setting to use it. I believe I've also seen this "webview" (maybe this isn't what you mean?) in other apps as well.

Edit: Bit of research suggests that this may be the "Custom Tab API"[1] which might be something different, perhaps a newer API?

1. https://www.reddit.com/r/firefox/comments/zkbf8v/enable_fire...