Firebase.failed() returns 0 after Firebase.begin(), but returns 1 while reading or writing from/to firebase from ESP8266
Apparently the Firebase's fingerprint has changed recently and has caused the above issue! 

from here get the latest finger print and change it in the ...\Arduino\libraries\firebase-arduino-master\src\FirebaseHttpClient.h

In the file mentioned above: update the value of static const char kFirebaseFingerprint[] to the new one mentioned above. 

P.S. One issue going forward for me(planing on implementing it on a long term basis) is how to be ahead of this issue in the future, so that I can get the data uninterrupted!