Firebase does not support LinkedIn natively, but it does allow the creation of a custom token. This method is used mainly when the authentication mechanism is actually performed somewhere else, either on your own server or another SaaS provider. In this case, you will be using Auth0 to provide the LinkedIn authentication. You could have performed the Facebook, Twitter, or Google authentication via Auth0 as well. However, it's recommended to use Firebase as much as possible as it's simpler to use. This is not only due to cost-related reasons, but also for the reduction of the number of integration points and code complexity.