Firebase is great but its more of a tool to build real time applications and chat can be one of the real time application use case but its a tool and not a final solution for chat.
You can build a mvp quickly using Firebase and you will have to still spent lot of time on going through the APIs, designing UI and build multiple small small things eg:
and although small but many other things which will take a lot of time when you will start to scale the system and add more capabilities to provide whatsapp like experience to your end users.
For a scalable and long term solution, you can use 'chat as a service' solution provided by many companies where you will get a final Chat solution with Backend APIs, Chat SDK and UI toolkit with all the whatsapp like features along with UI interface in-built and you wouldn't need to spend anything more than 15-20 mins to get chat up and running. Have a look at applozic.com and similar companies.
PS: I am cofounder of Applozic