We have limited cross compilation support for non-mobile platforms, see
https://firebase.google.com/docs/cpp/setup#libraries_desktop
The network stack, file I/O and a load of #ifdefs would need to be ported to support Xbox and PS4 or other platforms (Switch, etc.). Also, if you decided to port the C++ and maintain a fork you'll need to review the developer agreements for their respective SDKs to make sure you don't violate any agreements with them. Let us know if you're going to proceed, we could then discuss any restructuring required in the various platform abstraction layers in the SDK to facilitate this.
Cheers,
Stewart