Given a firebase, and populated users, you can start a login request creating a custom token for that access, and then checkout it on client side with web SDK

passport-firebase-admin-sdk.coffee

passport = require('passport')
passport_custom = require('passport-custom')

getAuth = (req) -> new Promise (resolve, reject) -> resolve req.query['email']
              
passport.use 'firebase-admin-login', new passport_custom (req, done) ->
                onErr = (error) ->
                        console.log error
                        done error.code, null, message: error.message
                        
                onUser = (userRecord) ->
                        uid = userRecord.toJSON().uid
                        firebase_admin.auth()
                                .createCustomToken(uid)
                                .then (customToken) ->
                                        console.log customToken
                                        done null, uid, customToken: customToken
                                .catch onErr
                                
                getAuth(req).then (email) ->
                        firebase_admin.auth()
                                .getUserByEmail(email)
                                .then onUser
                                .catch onErr
                        .catch onErr