I suspect a long pathname problem. Windows only handles paths up to 260 characters long. Depending on where your iOS project is located, that file might be beyond that limit. If it's not the case already, try putting your Xcode project in C:\iOS (that directory was created by the builder's install program, and its purpose was just that, to hold Xcode projects as close as possible to the drive root so as to avoid that).

If your Xcode project is already in C:\iOS, then it's another problem and I'll need to investigate.

You're almost there: all your files compiled successfully. Now the error is at the linking stage.

That's a strange error message: it says that a static library exports the same symbol twice, in two different files. It might mean it wasn't built correctly: possibly one in each of those two pair of files was supposed to be a symbolic link to the other. If the compiler didn't figure that out, then it was compiled twice. I think I have something to fix here. Yes, I'd be happy if you can prepare a small Xcode project with only the Firebase framework that demonstrates the problem.

Well, considering the workload here, I'm very glad you figured out the problem, and that it was not something to fix again.

Happy building :-)