Firebase and FusionAuth are two identity management platforms that offer different features, pricing models, and approaches. While they share some similarities, such as allowing users to register and login, customize transactional emails, and implement quickly on various platforms, their differences lie in cost, scalability, customization options, and support for single-tenant solutions. FusionAuth is free for unlimited users, offers more flexible email customization, supports localization, configurable password encryption, and enterprise identity unification, whereas Firebase has a more limited set of features and is part of the Google ecosystem, which may not cater to smaller clients with unique needs.