Use of unresolved identifier 'FIRDatabase' when using Firebase

You need to add

  pod 'Firebase/Database'

to your pod file as explained here

https://firebase.google.com/docs/database/ios/start

Here you can find which pod includes which feature Pods and Features

What worked for me, in addition to adding pod 'Firebase/Database', was (which is missing from the current docs):

import FirebaseDatabase

Tested with Xcode 7.3.1, CocoaPods 0.39.0 & 1.0.1 on macOS 10.11.5. To make sure you're doing this correctly, please make sure your Podfile.lock looks something like:

PODS:
  ...
  - FirebaseDatabase (3.0.1):
    - FirebaseAnalytics (~> 3.2)
  ...

P.S. I ran into CocoaPods issues that forced me to delete the derived data folder, so try that as well.