I already did some projects on Firebase with NodeMCU (ESP8266) .I got run time errors but mine's worked fine .U should check with your libraries . also tested over more than 2 libraries . In the code above u mentioned is not declared the FIREBASE_HOST,FIREBASE_AUTH, WIFI_SSID and WIFI_PASSWORD variable values may be u removed for privacy .The serial gps is not a big deal.U should check ur firbase with the given example of the particular header file or library .I didnt used the ArduinoJson.h library at all ....