Hi Richard.
The most common web framework for nodejs is
Express It is very simple and easy to get started for basic routes.
Personally I find it's core to be too minimal and I prefer something more comprehensive
hapi is my favorite node framework
and it comes with a lot of essential features built in.
Both hapi and Express has large user base and a lot of plugins for extra features (express uses the term middleware for plugins though)
Since you mentioned firebase and I assume you need some minimal layer to access it you might wanna check
GraphQL (I never personally used it).
GraphQL seem to be a hot trend now for designing api endpoints. It is a lot more flexible and powerful than traditional REST.
Though you can use GraphQL with Express and hapi it
Apollo is specifically designed to be used with GraphQL
Kind Regards,
Yoni