The closest you'd get to a "firefox webview" is by installing a Magisk module/non-Google ROM that replaces it with a stub application that forwards any "open system webview" call to your regular browser (so it works like clicking an http link). In the past you could just install that stub on any old Android device, but Google locked it down by letting vendors set a signature that a system webview would need to adhere to[0] (as well as a ton of other restrictions).
To be clear, I am not referring to what happens if you just click an http link in an app and the app just calls the browser. That setting respects browser choice. I am referring to the WebView[1] component, which a lot of apps use as a substitute for launching the browser and relies on an underlying app called the Android System WebView[2] to render the pages. For a legitimate usecase; in chat applications (ie. Discord), where a user might visit a website only to then want to immediately go back to the chat itself, the app instead just creates a WebView and... that one is always Chromium, and is nowadays enforced by vendor settings (so it can't be changed without root). In less legitimate usecases, this is done for tracking reasons.
(Another big user of the WebView component for this was Facebook Messenger, but Facebook has switched to using an internal version of this webview component, for reasons only understood by Facebook and it effectively acts as a shadow "fourth internet browser" that is in practice a somewhat outdated version of Chromium.)
[0]: https://chromium.googlesource.com/chromium/src/+/HEAD/androi...
[1]: https://developer.android.com/reference/android/webkit/WebVi...
[2]: https://play.google.com/store/apps/details?id=com.google.and...