How to mixin reactfire to es6 react class. More info: https://github.com/firebase/reactfire/issues/38#issuecomment-192963227

ReactFire-ES6-ReactMixin.js

import React, { Component } from 'react'
import Firebase from 'firebase'
import ReactFireMixin from 'reactfire'
import reactMixin from 'react-mixin'

const ref = new Firebase('https://<APPNAME>.firebaseio.com/users')

class UsersList extends Component {
  constructor (props, context) {
    super(props, context)
    this.state = {
      users: []
    }
  }

  componentDidMount () {
    this.bindAsArray(ref, 'users')
  }

  renderUser (user) {
    return <p>{user.name}</p>
  }

  render () {    
    return (
      <div>
        {this.state.languages.map(this.renderLanguage)}    
      </div>
    )
  }
}

reactMixin(UsersList.prototype, ReactFireMixin)

export default UsersList