Alright, let's get started! First things first, make sure you've got these installed and ready to go on your machine:

• Node.js and npm (npm comes along with Node.js, so it's a two-for-one deal).
• Firebase CLI (To get this, just run npm install -g firebase-tools).

And don't forget to initialize your Firebase project by running firebase init from your project's root folder.

Using the Firebase CLI, create a new Cloud Functions project if you haven't done so already.

firebase init functions

This command will create a new functions directory in your project root with some sample code for an HTTP and a background function. Head over to this directory:

cd functions

Next, install the Firebase Admin SDK and Firestore SDK by running:

npm install firebase-admin @google-cloud/firestore

Now, let's define your cloud function that processes Firestore data in bulk. Create an index.js file (or update the existing one) and add the following code:

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

exports.bulkProcess = functions.https.onRequest(async (req, res) => {
    const db = admin.firestore();

    // process your data here
});

You can perform batch operations on Firestore data using Firestore's batch() method. Add this code to your cloud function:

const bulkProcess = functions.https.onRequest(async (req, res) => {
    const db = admin.firestore();
    
    const batch = db.batch();
    const docsSnapshot = await db.collection('YourCollectionName').get();
    
    docsSnapshot.forEach(doc => {
        const docRef = db.collection('YourCollectionName').doc(doc.id);
        batch.update(docRef, { /* your update data */ });
    });
    
    await batch.commit();

    res.status(200).send('Operation completed successfully');
});

This code fetches all the documents from 'YourCollectionName', updates them, and then commits the batch operation. Replace 'YourCollectionName' with the name of your own collection, and replace { /* your update data */ } with the actual data you want to update.

After defining the function, it's time to deploy it to the Firebase cloud. You can do this by running the following command:

firebase deploy --only functions

Once the function is deployed, you can call it to process your Firestore data in bulk.

Keep in mind that processing large amounts of data may take some time and might exceed the maximum execution time for cloud functions. Consider splitting the work into smaller batches or using a distributed count pattern depending on your use case.