It appears that if the Privacy Badger chrome extension from the EFF is installed, then it blocks Firebase on the client.
Specifically these two scripts get blocked by the client:
<script src="https://cdn.firebase.com/js/client/2.2.7/firebase.js"></script>
<script src="https://cdn.firebase.com/libs/reactfire/0.4.0/reactfire.min.js"></script>
Is there any way to get Firebase to work with client browsers that want to protect their privacy with Privacy Badger (https://www.eff.org/privacybadger)