The problem is the site is not sending any (useful) HTML during the first HTTP request. Therefore the browser has to wait for the JS to render the page before it can even think about displaying anything.

The fact the site then loads content from firebase is obviously a problem, but it's not so much firebase's problem as it is of the codebase itself - in an ideal world it should be rendering the initial HTML to send to the client on the server.

I have seen React based websites with server side rendering work with firebase with much better performance.