Alright, let's get started by creating a Firebase project. Head over to the Firebase console and sign in with your Google account. Here's what you need to do:

• Click on the "Go to Console" link on the Firebase website.
• Hit the "Create a Project" button.
• Give your project a name, agree to the terms and conditions, and then click "Continue."
• Follow the additional setup steps.
• Finally, click the "Create Project" button.

Once your project is ready, you'll be taken to the Firebase console dashboard.

Next up, we need to enable multi-tenant authentication in the Firebase console.

• In your dashboard, go to the "Authentication" section.
• Select "Sign-in Method" from the options.
• On this page, enable the Email/Password sign-in method.

To use Firebase in your app, you'll need to install the Firebase SDK. If you're using Node.js, you can do this with npm. Just run the following command:

npm install --save firebase-admin

After installing the SDK, it's time to initialize Firebase with your project configuration.

• Go back to your Firebase dashboard and grab the SDK configuration snippet from the project settings.
• Use this information to initialize Firebase in your project.

It usually looks something like this:

var admin = require("firebase-admin");

var serviceAccount = require("path/to/serviceAccountKey.json");

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount),
  databaseURL: "https://<DATABASE_NAME>.firebaseio.com"
});

In Firebase Authentication, each tenant has a unique ID that should be used when authenticating users.

On the client-side, when a user wants to authenticate, they'll need to provide the tenant ID. You can do this using the tenantId property of the auth instance.

On the server-side, after setting the tenantId, you'll use the usual signInWithEmailAndPassword method.

Here's a Firebase JavaScript example:

firebase.auth().tenantId = '<TENANT_ID>';
firebase.auth().signInWithEmailAndPassword(email, password)

This ensures that the right user is authenticated with the correct tenant.

After the user is authenticated, it's crucial to validate the tenant on the server-side. You can do this by decoding and verifying the Firebase token on each request:

admin
  .auth()
  .verifyIdToken(idToken)
  .then((decodedToken) => {
    let uid = decodedToken.uid;
    let tenantId = decodedToken.firebase.tenant; 
  }).catch((error) => {
    // Handle error
  });

The tenantId in the decoded token should match your tenant's unique ID. If it doesn't, you should reject the request as unauthorized.

This is a straightforward way to implement Firebase multi-tenant authentication. Depending on your setup, you might need to take extra steps to properly separate data between tenants.