No way to delegate Firebase token with v8 web SDK.

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Using @nikko blog post: Firebase and Auth0 v8 from legacy v7 delegation | by Nikko Ambroselli | Medium
for inspiration I came up with this example using a Firebase cloud function to delegate the Auth0 token since that has been removed on Auth0 side. To create a custom token in Firebase function, you need to generate a service account JSON file. In your firebase console, go to Project Settings > Service Accounts and then click “Generate New Private Key”. Save that generated json file in your functions directory as service-account.json and deploy. You also need to create a new Non-Interactive client in your Auth0 account and use that client’s ID on the back-end.

Here’s what the firebase functions index.js looks like:

    const admin = require('firebase-admin');
    const cors = require('cors')({origin: true});
    const auth0 = require('auth0-js');

     var auth0Web = new auth0.WebAuth({
        domain:       'YOURAUTH0DOMAIN',
        clientID:     'YOURCLIENTID'
     });
    
    var serviceAccount = require("./service-account.json");
    admin.initializeApp({
      credential: admin.credential.cert(serviceAccount),
      databaseURL: "https://YOURPROJECTID.firebaseio.com"
    });
    
    exports.delegateToken = functions.https.onRequest((req, res) => {
    cors(req, res, () => {
      let userId = req.body.userId;
      const accessToken = req.headers.authorization.split('Bearer ')[1];

 auth0Web.client.userInfo(accessToken, function(err, user) {
      if (err) {
        console.log(err)
        res.status(403).send('Unauthorized');
      } else {
        if (userId == user.user_id) {
    
      admin.auth().createCustomToken(userId)
      .then(function (customToken) {
        res.send(customToken)
      })
      .catch(function (error) {
        console.log('Error creating custom token:', error)
      })

    } else {
          res.status(403).send('Unauthorized');
        }
      }
    });

    });
    });

And on your front end, just like you parseHash normally after a login, I do an ajax call to the firebase function with the Auth0 user ID which returns the custom token that we now use to sign in:

export const app = Firebase.initializeApp(fbconfig)
var result = auth0.parseHash(window.location.hash)
      if (result && result.accessToken) {
        var id_token = result.idToken;
        window.location.hash = ''
        auth0.getUserInfo(result.accessToken, function (err, profile) {
              if (err) {console.log(err) }
       
              // Get CustomFirebase Token
                axios({
                  method: 'post',
                  data: {
                    userId: profile.user_id
                  },
                  baseURL: functionsURL,
                  url: '/delegateToken',
                  headers: {
                       Authorization: 'Bearer ' + result.accessToken
                  }
                  })
                .then(response => {
                      app.auth().signInWithCustomToken(response.data).catch(function(error) {
                        console.log(error)
                      });
                })
            }
      }

Have limited testing on this, and don’t feel 100% comfortable storing the service-account.json file on the server, but this appears to be a solution that maybe others can take and improve - please let me know if you do. Not sure why something like this didn’t come from Auth0 directly, it appears they have moved on from supporting Firebase users.