一部を除いて、Next.js に限らず他の構成でも使えるはず。

最終的なクライアントのコード

import { Analytics, getAnalytics } from "firebase/analytics";
import { FirebaseApp, getApp, getApps, initializeApp } from "firebase/app";
import { Auth, connectAuthEmulator, getAuth } from "firebase/auth";
import { Firestore, connectFirestoreEmulator, getFirestore } from "firebase/firestore";
import { Functions, connectFunctionsEmulator, getFunctions } from "firebase/functions";
import { FirebaseStorage, connectStorageEmulator, getStorage } from "firebase/storage";

const isEmulator = () => {
  const useEmulator = process.env.NEXT_PUBLIC_USE_FIREBASE_EMULATOR;
  return !!(useEmulator && useEmulator === "true");
};

export const firebaseConfig = {
  apiKey: process.env.NEXT_PUBLIC_FIREBASE_API_KEY,
  authDomain: process.env.NEXT_PUBLIC_FIREBASE_AUTH_DOMAIN,
  projectId: process.env.NEXT_PUBLIC_FIREBASE_PROJECT_ID,
  storageBucket: process.env.NEXT_PUBLIC_FIREBASE_STORAGE_BUCKET,
  messagingSenderId: process.env.NEXT_PUBLIC_FIREBASE_MESSAGING_SENDER_ID,
  appId: process.env.NEXT_PUBLIC_FIREBASE_APP_ID,
  measurementId: process.env.NEXT_PUBLIC_FIREBASE_MEASUREMENT_ID,
};

if (!getApps()?.length) {
  initializeApp(firebaseConfig);
}

export const firebaseApp = getApp();
export const firebaseAnalytics = typeof window !== "undefined" ? getAnalytics() : undefined;
export const firebaseStore = getFirestore();
export const firebaseStorage = getStorage();
export const firebaseAuth = getAuth();
export const firebaseFuncions = getFunctions();

// emulatorを使う場合はここで設定
if (isEmulator()) {
  connectAuthEmulator(firebaseAuth, "http://localhost:9099");
  connectFunctionsEmulator(firebaseFuncions, "localhost", 5001);
  connectFirestoreEmulator(firebaseStore, "localhost", 8080);
  connectStorageEmulator(firebaseStorage, "localhost", 9199);
}

type FirebaseClient = {
  firebaseApp: FirebaseApp;
  firebaseStore: Firestore;
  firebaseStorage: FirebaseStorage;
  firebaseAuth: Auth;
  firebaseFuncions: Functions;
  firebaseAnalytics?: Analytics;
};

export const firebaseClient: FirebaseClient = {
  firebaseApp,
  firebaseAnalytics,
  firebaseStore,
  firebaseStorage,
  firebaseAuth,
  firebaseFuncions,
};

注意点

• 環境変数はクライアントサイドレンダリング時にも使えるように、名称をNEXT_PUBLIC_*にする
• サーバーサイドレンダリング時に実行されるとエラーが出るものには、typeof window !== "undefined"で判定する