store/index.js using firebase 3.x library

index.js

import { EventEmitter } from 'events'
import Firebase from 'firebase'

// ENTER YOUR FIREBASE URL BELOW
const db = Firebase.initializeApp({
  apiKey:        'YOUR_FIREBASE_APP_SECRET',
  databaseURL:   'https://YOUR_FIREBASE_APP.firebaseio.com/',
});
const categoriesRef = db.database().ref().child('categories')
const bookmarksRef = db.database().ref().child('bookmarks')
const store = new EventEmitter()

let categories = {}
let bookmarks = {}

db.database().ref().on('value', (snapshot) => {
  var bookmarkData = snapshot.val()
  if (bookmarkData) {
    categories = bookmarkData.categories
    bookmarks = bookmarkData.bookmarks
    store.emit('data-updated', categories, bookmarks)
  }
})

store.addCategory = (category) => {
  categoriesRef.update(category)
}

store.deleteCategory = (catName) => {
  // first check if an 'Uncategorized' category exists, if not, create it
  if (!('Uncategorized' in categories)) {
    categoriesRef.update({'Uncategorized': 'white'})
  }

  for (var key in bookmarks) {
    if (bookmarks[key].category === catName) {
      bookmarksRef.child(key).update({category: 'Uncategorized'})
    }
  }
  categoriesRef.child(catName).remove()
}

store.addBookmark = (bookmark) => {
  bookmarksRef.push(bookmark)
}

store.deleteBookmark = (bookmarkId) => {
  bookmarksRef.child(bookmarkId).remove()
}

export default store

Diff of the changes

diff --git a/app/store/index.js b/app/store/index.js
index dbb1992..a847157 100644
--- a/app/store/index.js
+++ b/app/store/index.js
@@ -2,15 +2,18 @@ import { EventEmitter } from 'events'
 import Firebase from 'firebase'

 // ENTER YOUR FIREBASE URL BELOW
-const db = new Firebase("https://YOUR_FIREBASE_APP.firebaseio.com/")
-const categoriesRef = db.child('categories')
-const bookmarksRef = db.child('bookmarks')
+const db = Firebase.initializeApp({
+  apiKey:        'YOUR_FIREBASE_APP_SECRET',
+  databaseURL:   'https://YOUR_FIREBASE_APP.firebaseio.com/',
+});
+const categoriesRef = db.database().ref().child('categories')
+const bookmarksRef = db.database().ref().child('bookmarks')
 const store = new EventEmitter()

 let categories = {}
 let bookmarks = {}

-db.on('value', (snapshot) => {
+db.database().ref().on('value', (snapshot) => {
   var bookmarkData = snapshot.val()
   if (bookmarkData) {
     categories = bookmarkData.categories

Thanks very much for this! Please also note that you'll need to take the following steps in order for this to run successfully:

1. Clone repository
2. Update the Firebase version to 3.2.1 in your package.json
3. Update the /store/index.js file as specified in above file
4. Save
5. npm install
6. webpack
7. npm start

At least that's how I got all of it working :)