if you're within Node:

https://github.com/googleapis/nodejs-firestore/blob/master/dev/src/index.ts#L978

/**
* Retrieves multiple documents from Firestore.
*
* @param {...DocumentReference} documents - The document references
* to receive.
* @returns {Promise<Array.<DocumentSnapshot>>} A Promise that
* contains an array with the resulting document snapshots.
*
* @example
* let documentRef1 = firestore.doc('col/doc1');
* let documentRef2 = firestore.doc('col/doc2');
*
* firestore.getAll(documentRef1, documentRef2).then(docs => {
*   console.log(`First document: ${JSON.stringify(docs[0])}`);
*   console.log(`Second document: ${JSON.stringify(docs[1])}`);
* });
*/

This is specifically for the server SDK

UPDATE: "Cloud Firestore [client-side sdk] Now Supports IN Queries!"

https://firebase.googleblog.com/2019/11/cloud-firestore-now-supports-in-queries.html

myCollection.where(firestore.FieldPath.documentId(), 'in', ["123","456","789"])

They have just announced this functionality, https://firebase.googleblog.com/2019/11/cloud-firestore-now-supports-in-queries.html .

Now you can use queries like, but mind that the input size can't be greater than 10.

userCollection.where('uid', 'in', ["1231","222","2131"])

In practise you would use firestore.getAll like this

async getUsers({userIds}) {
    const refs = userIds.map(id => this.firestore.doc(`users/${id}`))
    const users = await this.firestore.getAll(...refs)
    console.log(users.map(doc => doc.data()))
}

or with promise syntax

getUsers({userIds}) {
    const refs = userIds.map(id => this.firestore.doc(`users/${id}`))
    this.firestore.getAll(...refs).then(users => console.log(users.map(doc => doc.data())))
}