First, ensure you have Node.js installed on your machine. You'll also need to set up a new project directory and initialize it with npm:
```bash
mkdir stripe-to-firestore
cd stripe-to-firestore
npm init -y
```

Install the Stripe and Firestore client libraries:
```bash
npm install stripe @google-cloud/firestore
```

Create a new file, `index.js`, and start by requiring the Stripe library and initializing it with your Stripe secret key. This will allow you to make authenticated requests to Stripe's API:
```javascript
const stripe = require('stripe')('your-stripe-secret-key');
```

Set up Firestore in your `index.js` file. You need to authenticate using a service account key file, which you can download from the Google Cloud Console:
```javascript
const { Firestore } = require('@google-cloud/firestore');
const firestore = new Firestore({
projectId: 'your-project-id',
keyFilename: 'path-to-your-service-account-file.json',
});
```

Use Stripe's API to retrieve data. For example, to fetch all customers:
```javascript
async function fetchStripeCustomers() {
const customers = await stripe.customers.list();
return customers.data;
}
```

Depending on your needs, you may need to transform the data before saving it to Firestore. This might involve mapping data fields or cleaning up the data format:
```javascript
function transformData(customers) {
return customers.map(customer => ({
id: customer.id,
email: customer.email,
name: customer.name,
// Add more fields as needed
}));
}
```

Finally, save the transformed data to Firestore. You can loop through the data and use Firestore's `set` or `add` methods to store each item:
```javascript
async function saveToFirestore(customers) {
const collectionRef = firestore.collection('customers');
for (const customer of customers) {
await collectionRef.doc(customer.id).set(customer);
}
}

async function main() {
const stripeCustomers = await fetchStripeCustomers();
const transformedData = transformData(stripeCustomers);
await saveToFirestore(transformedData);
console.log('Data transferred successfully.');
}

main().catch(console.error);
```

With this setup, you will have created a simple script to transfer data from Stripe to Google Firestore. This approach requires maintaining your code and handling any errors or changes in the APIs over time, but it provides full control over the data transfer process.