Since version 5.3.0 you can choose either Crashlytics (recommended) or the older Firebase Crash Reporting (which doesn't really work on iOS). You will be prompted during installation of the plugin to enable either Crashlytics or Crash Reporting (both are off by default).
Note that if you want to use Crashlytics, make sure your firebase.nativescript.json file has "crashlytics": true and "crash_reporting": false,
then remove the platforms folder so these changes are picked up.