Firebase Firestore Query get One result

Your query returns a QuerySnapshot (see docs). You can access the docs as an array via the docs property; each doc has a data() method:

export const findUserByEmail = email => {
  return firestore()
    .collection('users')
    .where('email', '==', email.toLowerCase())
    .get()
    .then(querySnapshot => {
      if(!querySnapshot.empty) {
        const user = querySnapshot.docs[0].data()
        // rest of your code 
      }
    })
}