Thanks, but opening the package directly in Xcode fails because of
error: unexpected service error: build aborted due to an internal error: planningFailed("multiple configured targets of \'Firebase\' are being created for macOS")
that is described in detail here.
(Created [SR-11321] [SwiftPM] generate-xcodeproj does not generate relative search paths correctly · Issue #4676 · apple/swift-package-manager · GitHub for the issue with generate-xcodeproj)