This script may no longer work. Have a look at its (more official) replacement: https://github.com/firebase/firebase-tools/tree/master/scripts/examples/hosting/update-single-file
This utility script deploy a single local file to an existing Firebase Hosting site. Other files that are already deployed are left unmodified.
The difference with firebase deploy is that this script does not require you to have a local snapshot of all hosted files,
you just need the one file that you want to add/update.