If you're as confused as I am that there's an API for custom options, yet Google still requires a file named GoogleService-Info.plist in your app, here's some swizzling that fixes that for ya. All Swift :)

FirebaseCoordinator.swift

class FirebaseCoordinator {
    static let shared = FirebaseCoordinator()
    static let initialize: Void = {
        /// We modify Google Firebase (and eventually Analytics) to load the mac-specific plist at runtime.
        /// Google enforces that we have a file named "GoogleService-Info.plist" in the app resources.
        /// This is unfortunate since we need two different files based on iOS and Mac version
        /// One solution is a custom build step that copies in the correct file:
        /// https://stackoverflow.com/questions/37615405/use-different-googleservice-info-plist-for-different-build-schemes
        /// However, this is basically impossible since Catalyst doesn't set any custom build variables, so detection is extremely difficult.
        /// We swizzle to modify the loading times.
        /// There is also official API: FirebaseApp.configure(options: firebaseOptions()) however this is too late for Google Analytics.
        #if targetEnvironment(macCatalyst)
        if !FirebaseOptions.configurePlistPath() {
            print("Warning: Overriding Mac-specific plist failed in Firebase. Check plist override code")
        }
        #endif
        FirebaseApp.configure()
        #if DEBUG
        print("Firebase Initialized. isDataCollectionDefaultEnabled: \(FirebaseApp.app()!.isDataCollectionDefaultEnabled)")
        #endif
    }()

    private init() { }

    static func configure() {
        FirebaseCoordinator.initialize
    }
}

extension FirebaseOptions {
    /// Modifies the Google Firebase plist path to load GoogleService-Info-Mac if on Mac Catalyst.
    static func configurePlistPath() -> Bool {
        guard let klass = object_getClass(self) else { return false }
        let sel = NSSelectorFromString("plistFilePathWithName:")
        var origIMP : IMP? = nil
        let newHandler: @convention(block) (AnyObject, String) -> String = { blockSelf, name in
            typealias ClosureType = @convention(c) (AnyObject, Selector, String) -> String
            let callableIMP = unsafeBitCast(origIMP, to: ClosureType.self)
            return callableIMP(blockSelf, sel, name + (IsMacCatalyst() ? "-Mac" : ""))
        }
        guard let method = class_getClassMethod(klass, sel) else { return false }
        origIMP = class_replaceMethod(klass, sel, imp_implementationWithBlock(newHandler), method_getTypeEncoding(method))
        return origIMP != nil
    }
}

// Future code: Currently Mac Catalyst doesn't support Google Analytics yet.
extension FirebaseCoordinator {
    /// Modifies the path that Google Analytics uses to load the plist
    static func configureGoogleAnalyticsPlistPath() -> Bool {
        // id +[APMInfoPlistFileUtil googleServiceInfoPlistPath](id a1, SEL a2)
        guard let klass = object_getClass(NSClassFromString("APMInfoPlistFileUtil")) else { return false }
        let selector = NSSelectorFromString("googleServiceInfoPlistPath")
        let newHandler: @convention(block) (AnyObject) -> String = { blockSelf in
            let newName = "GoogleService-Info" + (IsMacCatalyst() ? "-Mac" : "")
            return Bundle.main.path(forResource: newName, ofType: "plist") ?? ""
        }

        guard let method = class_getClassMethod(klass, selector) else { return false }
        let origIMP = class_replaceMethod(klass, selector, imp_implementationWithBlock(newHandler), method_getTypeEncoding(method))
        return origIMP != nil
    }
}