Firebase multiple "apps" - client side and server side

So, I wanted to add another firebase app into the project. I already had one for SSR (firebase-admin), so server could pre-render sites. Now I want to add firebase@8, so I can start using it on client side, and later apply firestore rules to it. The problem is, when I'm trying to use it I get this error:

/node_modules/@google-cloud/storage/build/src/bucket.js:22:0
Module not found: Can't resolve 'fs'

Import trace for requested module:
./node_modules/@google-cloud/storage/build/src/index.js
./node_modules/firebase-admin/lib/storage/storage.js
./node_modules/firebase-admin/lib/app/firebase-namespace.js
./node_modules/firebase-admin/lib/default-namespace.js
./node_modules/firebase-admin/lib/index.js
./firebase/config.js
./firebase/createDocument.js
./components/form/Form.js
./pages/opinia/index.js

https://nextjs.org/docs/messages/module-not-found

Here is my config file:

import * as admin from 'firebase-admin';
import firebase from 'firebase';
import 'firebase/firestore';

// initialize firebase on CLIENT
const firebaseConfig = {
  apiKey: process.env.FIREBASE_KEY,
  authDomain: process.env.FIREBASE_DOMAIN,
  projectId: process.env.FIREBASE_ID,
  storageBucket: process.env.FIREBASE_STORAGE_BUCKET,
  messagingSenderId: process.env.FIREBASE_MESSAGING_SENDER_ID,
  appId: process.env.APP_ID,
};

export const clientApp = initializeApp(firebaseConfig);

// firestore
export const projectFirestoreClient = firebase.firestore(clientApp);

// timestamp
export const timestampClient = firebase.firestore.Timestamp;

// initialize firebase on SERVER
if (!admin.apps.length) {
  const serviceAccount = require('./firebase-adminsdk.json');

  admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
  });
}

// firestore
export const projectFirestoreAdmin = admin.firestore();

// timestamp
export const timestampAdmin = admin.firestore.Timestamp;

So, my question is, how do I fix this error? And what's causing it? Thanks in advance.