FirebaseMessagingHelper.java

public class FirebaseMessagingHelper extends FirebaseInstanceIdService {

    static public String token;

    @Override
    public void onCreate() {
        super.onCreate();

        token = FirebaseInstanceId.getInstance().getToken();
        if(token != null) Timber.d("Token %s", token);
    }

    @Override
    public void onTokenRefresh() {
        token = FirebaseInstanceId.getInstance().getToken();
    }
}

Não esquece de colocar no Manifest

        <service android:name=".services.FirebaseMessagingService">
            <intent-filter>
                <action android:name="com.google.firebase.MESSAGING_EVENT" />
            </intent-filter>
        </service>
        <service android:name=".services.FirebaseMessagingHelper">
            <intent-filter>
                <action android:name="com.google.firebase.INSTANCE_ID_EVENT" />
            </intent-filter>
        </service>