The User class is a property set class, something we use internally that's sort of akin to a Python dictionary, but with type safety.
So the User class defines various static constants (such as FirstName) that define the properties; but to retrieve the actual values from a given user object, you need to use an intermediate function like get or getOrDefault:
roster = system.roster.getRoster("ttf_coldroom").getUsers()
for userObject in roster:
logger.info(userObject.get(userObject.FirstName))