Although current version of firebase hosting is optimized for next and angular, I don't see why I can't use the same to run pure backend services written in any language if google cloud run is used in the background. If it's possible to use your own dockerfile and mention a port for listening to traffic, using your own build process within the dockerfile, I can setup custom backend with the simplicity of firebase.

I would get control over binaries in my backend, like puppeteer needs chrome etc.
I would get auto-ssl, auto scaling , load balancing capabilities
This can be a drop in replacement for something like digitalocean.