firebaseLazyImportUsage.js

// before:
import firebase from 'what/ever/firebase'

const {auth} = firebase

export function signIn({email, password}) {
  auth.signInWithEmailAndPassword(email, password)
    .then((user) => {
      // ...
    })
}
// ---

// after:
import firebase from 'what/ever/firebase'

export function signIn({email, password}) {
  firebase().then(({auth}) => {
    auth.signInWithEmailAndPassword(email, password)
      .then((user) => {
        // ...
      })
  })
}
// ---

// or a little less verbose version
function withFirebase(callback) {
  return firebase().then(callback)
}

// ...
import {withFirebase} from 'what/ever/firebase'

export function signIn({email, password}) {
  withFirebase(({auth}) => {
    auth.signInWithEmailAndPassword(email, password)
      .then((user) => {
        // ...
      })
  })
}