It looks like that endpoint is the method Firebase uses to deliver a non-WebSocket fall-back when WebSocket connections fail (?.lp -> “Long Polling” endpoint), and that uses JSONP to deliver the data instead. In that scenario the JSONP ultimately gets included as a <script>, but only for (I believe) the purpose of delivering data when cross-domain policies in web browsers prohibits .json from being loaded directly.
The good news is that endpoint doesn’t get requested at all in the happy path where the WebSocket connects OK.
This leaves me thinking Firebase might still be fine to use as a WebSocket-only back-end, let’s say if you force it to not allow the long polling fall-back (firebase.database.INTERNAL.forceWebSockets()).
Re: readability of the libraries in the submitted zip, wonder if webpack-importing the Firebase requisites into a vendor.js would be preferable / make it fine? Since the libraries they provide on their static CDN appear to be minified e.g. https://www.gstatic.com/firebasejs/4.5.0/firebase-database.js