Firebase.Auth (which you use) is linked with UIWebView also.
To be sure, create a new project. At first include FirebaseAnalytics only. Compile, rename ipa to zip, extract executable file B4iExample.
Open it in text editor and search by UIWebView. Nothing.
Add Firebase.Auth, do the same. A lot of UIWebView.
BTW, libiFirebase.Auth is 'clear'. But there are references to frameworks.