Hello Peter,
The problem is, of course, the time... I am busy with some customer's projects, also with my own ones... I have no time, for sure, to start to read about Firebase in a deep way right now, Peter. Maybe in the future, but for now maybe we can go ahead in a step by step way...
In principle, whatever thing that you can see working in an HTML file... must work in AB, because our AB apps are also HTML... of course we need what happen in the HTML file, what we do in that, in order to maybe "translate" and do the same thing in AB.
You mention that the database that you want to use don't provide JSON, but, you don't mention what is the format that the database provide... maybe you can use that format directly in AB (again, if the HTML file do it... in principle we can also do it...).
Another possible solution is to use a "bridge" between the database and our app. That is, we can prepare an app's server script, who deal with the database, and finally provide to the app the right data, maybe already converted to JSON, if we wanted that.
So please, try the above, and, of course, if you find any specific error or problem (this is important, that is, to be specific), don't hesitate to post it here, and we will try to help you as bes as possible.
P.S. I want to try your provided app XML, but, can't open it properly... please, take a look... and, if once you be sure that the code is correct, post it here: if then don't work... maybe you need to send me this specific XML (app) using my E-Mail address...