VLC has Chromecast support in the development version, and has had it for over a year now. They had to reverse-engineer the protocol. I think the UI is a bit cludgey but it works.
> This framework implements the Google Cast APIs so they can be used in macOS apps. Google provides an official SDK but it is only for iOS and closed source.
So this means other applications like Firefox, VLC etc... would have to reverse engineer the protocol.