The Firebase ML Vision SDK is now deprecated.
These docs describe how, as an alternative to the deprecated SDK, you can call Cloud Vision APIs using Firebase Auth and Firebase Functions to allow only authenticated users to access the API.
These codelabs describe how you can train and deploy your Custom Models.
UPDATE: On June 3, 2020, we started offering ML Kit's on-device APIs through a new standalone SDK. Cloud APIs, AutoML Vision Edge, and custom model deployment will continue to be available via Firebase Machine Learning. Learn more