After updating to XCode 8 GM(Swift 3)
Firebase started logging these error's in the Console:-
FIRInstanceID/WARNING STOP!! Will reset deviceID from memory.
FIRInstanceID/WARNING Failed to fetch default token Error Domain=com.firebase.iid Code=6 "(null)"
FIRInstanceID/WARNING Failed to retrieve the default GCM token after 5 retries
Apparently Enabling your app's KeyChain Sharing in Capabilities does the trick!
After enabling your capabilities , app.entitlements should look something like this:-
Also See:- Firebase Group Talk