First things first, make sure you've got the Firebase CLI (Command Line Interface) installed. Also, you should have a Firebase project set up with Firebase Hosting enabled.

Firebase Functions are great for server-side rendering, and you'll need the Express library to set up your Firebase functions to respond with SEO-friendly URLs.

npm install firebase-functions@latest express --save

Run this command to get Firebase Functions going with Javascript:

firebase init functions

Head over to your functions folder, where you'll find the index.js file. Import Express like this:

const functions = require('firebase-functions');
const express = require('express');
const app = express();

Next, set up a simple function to respond at the root of your URL.

app.get('/', (req, res) => {
  res.send('Hello from Firebase!');
});

Finally, export your app as a function that responds to HTTP requests:

exports.app = functions.https.onRequest(app);

To create SEO-friendly URLs, define a clean URL structure for each page of your site using Express routes.

app.get('/user/:id', (req, res) => {
  res.send(`This is the page of user with id ${req.params.id}`);
});

The :id is a URL parameter that you can access within your function through req.params.

Add a rewrite in your firebase.json file to link your Express app with your website's URL.

{
  ...
  "hosting": {
    ...
    "rewrites": [
      {
        "source": "**",
        "function": "app"
      }
    ]
  }
}

Now it's time to deploy your Firebase Function and Hosting files. During the deploy process, Firebase will connect your function with your hosting setup.

firebase deploy --only functions,hosting

After deployment, your SEO-friendly URLs are live. Google can now crawl your pages through those clean URLs, giving you a stronger SEO presence.