Execute create-next-app with Yarn or npx to bootstrap the example:

npx create-next-app --example with-firebase-authentication with-firebase-authentication-app
# or
yarn create next-app --example with-firebase-authentication with-firebase-authentication-app

Download the example:

curl https://codeload.github.com/vercel/next.js/tar.gz/canary | tar -xz --strip=2 next.js-canary/examples/with-firebase-authentication
cd with-firebase-authentication

Set up Firebase:

• Create a project at the Firebase console.
• Copy the contents of .env.local.example into a new file called .env.local
• Get your account credentials from the Firebase console at Project settings > Service accounts, where you can click on Generate new private key and download the credentials as a json file. It will contain keys such as project_id, client_email and client_id. Set them as environment variables in the .env.local file at the root of this project.
• Get your authentication credentials from the Firebase console under Project settings > General> Your apps Add a new web app if you don't already have one. Under Firebase SDK snippet choose Config to get the configuration as JSON. It will include keys like apiKey, authDomain and databaseUrl. Set the appropriate environment variables in the .env.local file at the root of this project.
• Go to Develop, click on Authentication and in the Sign-in method tab enable authentication for the app.

Install it and run:

npm install
npm run dev
# or
yarn
yarn dev

Deploy it to the cloud with Vercel (Documentation).

After deploying, copy the deployment URL and navigate to your Firebase project's Authentication tab. Scroll down in the page to "Authorized domains" and add that URL to the list.