I haven't been using Firefox for Android because I heard they don't have a WebView Implementation so the firefox browser has to be used beside the Chromium WebView meaning there's an attack surface of two browser engines. I also heard that the Firefox sandboxing and site isolation isn't very good between websites.
I've been using Vanadium WebView and browser because of that.
Android System Webview allows apps to display browser windows in the app rather than taking you to your web browser app. On Android, chromium is used for webview. If you use Firefox as a default browser, the remote attack surface increases because they're two different browsers with different security issues.
Site isolation enforces security boundaries around each site using the sandbox by placing each site into an isolated sandbox. Firefox doesn't have that feature so they're vulnerable to attacks like Spectre.