The current Swift client for the Real-time database is written in Objective-C with a Swift overlay.
In order to allow using Firebase from Swift across different platforms like Windows and Linux, it would be awesome with a native Swift version. Alternatively a Swift version bridged directly from the firebase-cpp-sdk using the new Swift C++ interoperability.

I did a port of the Objective-C code to Swift as an experiment, and I have it running on Linux just fine - together with the Auth-rewrite in Swift.

The (very experimental) PR is here: https://github.com/firebase/firebase-ios-sdk/pull/8670