tenant_auth.js

// Specify the unique ID of the tenant to authenticate.
firebase.auth().tenantId = 'TENANT_ID1';

// Usual Firebase sign-in logic. User will be authenticated
// with tenant-specific identity providers.
firebase.auth().signInWithEmailAndPassword(email, password)
  .then((result) => {
    const user = result.user;
    // user.tenantId is set to 'TENANT_ID1'.
  });