Mark Hudson, it IS json. Firebase can also import and export json data.
I also believe that for Lily Chen’s use-case what she built — while a basic CMS- it is one and it makes sense. I read the article you suggested reading, and much of it makes sense, what it’s missing though as an argument for it is the myriad of small clients who simply want nothing more than a way to edit their existing content and add a couple of posts here and there without having to pay $50/h to a developer to do it, $20 for a monthly CMS service that they find overly complex to work with. In the PHP world there is a great solution for that called CouchCMS, but this is not PHP and Firebase is a good free and secure solution for an ad-hoc CMS, not sure why you’d be against running it on prod. By now probably half your mobile apps connect to Firebase and more and more sites and web apps see it a viable solution.