On github i see this path to a file:
importScripts('/__/firebase/3.8.0/firebase-app.js');
importScripts('/__/firebase/3.8.0/firebase-messaging.js');
importScripts('/__/firebase/init.js');
What does the "/__/" stands for?
On Firebase Hosting, the Firebase SDK along with a script that initializes the SDK for the current project are automatically served from special reserved URLs. You can read more about it on this blog post.
This means all the libraries are inside root folder.