Well, they do say: "Firebase can be used to easily build multiplayer games!"
And by way of example, they offer "a fully-functional multi-player game of Tetris built entirely without server-side code".
But it's the "built entirely without server-side code" bit that would seem to rule out Firebase as a viable platform for industrial-strength secure multiplayer games, where you simply must have customised server-side code to handle such issues as cheating.
By way of illustration, their two-player Tetris example includes a Tetris.Board object in the client code which according to their comment /* Stores the state of a tetris board and handles drawing it */ - which in a proper multiplayer game is something that needs to be handled by the game server, otherwise what's to stop one player changing the state of the board to get an unfair advantage?
[UPDATE after further poking around the Firebase site]
To quote (my emphasis):
"For the most part, you really can write full apps without writing any server code and everything is amazing and there are kittens and bunnies everywhere. If this sounds too good to be true, here?s ?the catch?: In some cases, you may still need your own server.
"Firebase is great for storing and propagating data, but that?s not the only thing servers do. You might need server code to integrate with other APIs, send out emails to users, do heavy data processing, etc."
- and, of course, in multiplayer games, to handle player interactions so that such issues as security and latency are dealt with appropriately and effectively.
So, not a viable multiplayer gaming platform, IMO.