Use LinkedIn Sign In with Firebase

This sample is based off from this Instagram Sample

LinkedIn app's creation and setup

Register an LinkedIn app on LinkedIn for Developers.
Once Your app is created make sure you specify your app's callback URL in the list of Valid redirect URIs of your LinkedIn app. You should whitelist http://localhost:3030/linkedin-callback for local development.
Copy the Client ID and the Client Secret of your LinkedIn app and copy it into config_.json in place of the placeholders, and rename the file to config.json

Firebase app creation and setup

Create a Firebase project using the Firebase Developer Console.
For the web app: Copy the Web initialisation snippet from Firebase Console > Overview > Add Firebase to your web app and paste it in public/index.html in lieu of the placeholders (where the TODO(DEVELOPER) is located).
For the server: From the Firebase initialization snippet copy the apiKey value and paste it in config.json as the value of firebase.apiKey in lieu of the placeholder.

Create and provide a Service Account's keys:

Create a Service Accounts file as described in the Server SDK setup instructions.
Save the Service Account credential file as service-account.json

Deploy your security rules:

Run firebase use --add and choose your Firebase project. This will configure the Firebase CLI to use the correct project locally.
Run firebase deploy to deploy the security rules of the Realtime Database

Run the sample locally

You can run the sample web app locally by running the following in a command line window.

npm install
node app.js

Then open http://localhost:3030 in your browser.

Oauth 2 flows

When you click the Sign in with Linkedin button, it goes to localhost:8080/redirect on a new browser window
The server process this request and redirects to the linkedin oauth endpoint, https://www.linkedin.com/oauth/v2/authorization?response_type=code&client_id=${credentials.client.id}&redirect_uri=${req.protocol}://${req.get('host')}/linkedin-callback&state=${state}&scope=r_basicprofile
If the user has already granted the linkedin service to the web app, it will redirect back to the specified redirect uri. Otherwise, it will ask the uesr to grant the permissions the web app is asking for.
After the user granted the permissions and gets redirected back with the authorization code to the web server. i.e. localhost:3030/linkedin-callback?code=faskldjfioajglakjsdfagoijefaj
The web server will then use the authorization code to exchange for an access token from linkedin.
After the linkedin access token is retrieved, the web server will make a call to linkedin again with the access token to retrieve user profile data.
Now the web server has the LinkedIn access token and the user profile data, it then send those to firebase for creating an account in firebase.
Firebase will create the user and return a firebase access token back to the web app server.
The web app server then sends the firebase access token to the web browser, alone with javascript code that initializes firebase on the browser, and then closes the popup window it was launched when the sign in button was clicked.
Now the web browser has the firebase access token which can be used to identify the user, and it can use it to retrieve user profile.

LinkedIn Reference:

https://developer.linkedin.com/docs/oauth2

Note for LinkedIn mobile access token

Android LinkedIn SDK produces unusable access tokens

Mobile vs. server-side access tokens

It is important to note that access tokens that are acquired via the Mobile SDK are only useable with the Mobile SDK, and cannot be used to make server-side REST API calls.

Similarly, access tokens that you already have stored from your users that authenticated using a server-side REST API call will not work with the Mobile SDK.