Error running FirebaseDemo on nodeMCU
It does warn of some deprecations and such in the firebase libraries :
In member function 'const String&amp; FirebaseArduino::error()' warning: returning reference to temporary [-Wreturn-local-addr] return error_.message().c_str();

i also get some depreciation like
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseObject.cpp: In member function 'bool FirebaseObject::isNullString(const String&amp;) const':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseObject.cpp:67:57: warning: 'const char* ArduinoJson::Internals::JsonVariantCasts&lt;TImpl&gt;::asString() const [with TImpl = ArduinoJson::JsonVariant]' is deprecated (declared at /home/praz/Arduino/libraries/ArduinoJson-5.13.1/src/ArduinoJson/Deserialization/../JsonVariantCasts.hpp:28): use as&lt;char*&gt;() instead [-Wdeprecated-declarations]
   return variant.is&lt;const char *&gt;() &amp;&amp; variant.asString() == NULL;
                                                         ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseObject.cpp: In member function 'ArduinoJson::JsonVariant FirebaseObject::getJsonVariant(const String&amp;) const':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseObject.cpp:87:26: warning: 'ArduinoJson::JsonObject&amp; ArduinoJson::Internals::JsonVariantCasts&lt;TImpl&gt;::asObject() const [with TImpl = ArduinoJson::JsonVariant]' is deprecated (declared at /home/praz/Arduino/libraries/ArduinoJson-5.13.1/src/ArduinoJson/Deserialization/../JsonVariantCasts.hpp:23): use as&lt;JsonObject&gt;() instead [-Wdeprecated-declarations]
     json = json.asObject().get&lt;JsonVariant&gt;(start);
                          ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/throw_out_of_range.cpp:6:28: warning: unused parameter 'str' [-Wunused-parameter]
 void __attribute__((weak)) __throw_out_of_range(const char* str) {
                            ^
In file included from /home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseCloudMessaging.cpp:1:0:
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseCloudMessaging.h:82:79: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
   const void AddToJson(const FirebaseCloudMessage&amp; message, JsonObject&amp; json) const;
                                                                               ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseCloudMessaging.cpp:80:60: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
     const FirebaseCloudMessage&amp; message, JsonObject&amp; json) const {
                                                            ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/Firebase.cpp: In member function 'int FirebaseRequest::sendRequest(const string&amp;, const string&amp;, char*, const string&amp;, const string&amp;)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/Firebase.cpp:76:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/Firebase.cpp: In member function 'void FirebaseStream::startStreaming(const string&amp;, const string&amp;, const string&amp;)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/Firebase.cpp:89:48: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   analyzeError(&quot;STREAM&quot;, status, path_with_auth);
                                                ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'String FirebaseArduino::push(const String&amp;, const ArduinoJson::JsonVariant&amp;)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:65:79: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   int status = req_.get()-&gt;sendRequest(host_, auth_, &quot;POST&quot;, path.c_str(), buf);
                                                                               ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:65:7: warning: unused variable 'status' [-Wunused-variable]
   int status = req_.get()-&gt;sendRequest(host_, auth_, &quot;POST&quot;, path.c_str(), buf);
       ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'void FirebaseArduino::set(const String&amp;, const ArduinoJson::JsonVariant&amp;)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:94:65: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   req_.get()-&gt;sendRequest(host_, auth_, &quot;PUT&quot;, path.c_str(), buf);
                                                                 ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'void FirebaseArduino::getRequest(const String&amp;)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:101:60: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   req_.get()-&gt;sendRequest(host_, auth_, &quot;GET&quot;, path.c_str());
                                                            ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'virtual void FirebaseArduino::remove(const String&amp;)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:147:63: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   req_.get()-&gt;sendRequest(host_, auth_, &quot;DELETE&quot;, path.c_str());
                                                               ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'virtual const String&amp; FirebaseArduino::error()':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:201:33: warning: returning reference to temporary [-Wreturn-local-addr]
   return error_.message().c_str();
                                 ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp: In member function 'virtual void FirebaseHttpClientEsp8266::begin(const string&amp;)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp:47:50: warning: 'bool HTTPClient::begin(String, String)' is deprecated (declared at /home/praz/.arduino15/packages/esp8266/hardware/esp8266/2.4.2/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h:156) [-Wdeprecated-declarations]
     http_.begin(url.c_str(), kFirebaseFingerprint);
                                                  ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp: In member function 'virtual void FirebaseHttpClientEsp8266::begin(const string&amp;, const string&amp;)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp:51:80: warning: 'bool HTTPClient::begin(String, uint16_t, String, String)' is deprecated (declared at /home/praz/.arduino15/packages/esp8266/hardware/esp8266/2.4.2/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h:157) [-Wdeprecated-declarations]
     http_.begin(host.c_str(), kFirebasePort, path.c_str(), kFirebaseFingerprint);
                                                                                ^
Compiling library &quot;ESP8266HTTPClient&quot;

to fixed that, go to Preferences in Compiler Warnings choose as Default. it ignored all warning in depreciation, don't forget to update your library to get Firebase Fingerprint has changed on 2 Aug 2018.
Uploaded your sketch again, your should be get data like