Firebaser here -
If you missed the migration window to move from Fabric to Firebase, the best way forward is to set up a new Firebase project, add your desired apps to your Firebase project, and then follow the Crashlytics onboarding instructions to get Firebase Crashlytics working. The Firebase Crashlytics SDK is now the main supported Crashlytics SDK, and the old Fabric SDK will no longer report crashes past November 15th, as Misha noted.
Once you've been able to verify that a test crash has come through to your dashboard, you should be all set to include it in a live version that goes out to your users.