> [...] if you set [Firefox] as a default browser you will many apps open content in a Firefox powered webview. (Activity)
This is great because the Firefox webview allows to break it out in to a proper Firefox tab in the app. Also, the Firefox webview also uses extensions installed in Firefox app, e.g. uBlock Origin.
Not if the application uses Custom Tabs to start a web browsing activity.
https://developer.chrome.com/docs/android/custom-tabs/
Firefox fully supports this and if you set it as a default browser you will many apps open content in a Firefox powered webview. (Activity)
Apple can do the same. Not trivial but definitely possible.