Custom function of querying firebase and returning a list of documents?

Does anyone have an example of using a Custom Function to query a Firebase collection and return a list of documents that can then be used to populate a list (etc) in FF?

I am REALLY struggling!

For example, query a list of {users} from Firebase collection where their {lastName} = "Smith", returning the resulting list.

I get constant errors around data types/return types etc.

Thank you.