This is very well put together! The API feels very natural and has an obvious flow to it, easy to work with.
The vast majority of Firebase projects should have their needs more than covered by these endpoints/services. I doubt Google would mind if you were to take some of those lower-end customers off their hands ;)
In order to make this comment look less like a paid endorsement: I couldn't find the upstream sources or info on your pre-processing of the data endpoints (news, stocks, etc.). Sadly without this information they are quite useless.
I can see this going places.
Thanks for the comments. Yea so we used to say "Powered By X" to indicate what the APIs were backed by but it was a little hacky. We'll try to reintroduce something back into specify the "source". You can otherwise see all the source code for the services in a monorepo on GitHub https://github.com/micro/services. Would be interested to get feedback on a better way to make it available.