How to update boolean value in Firestore from a switch tile?

You could improve your updateNotifications() function to not have duplicate code:

 Future<void> updateNotifications() async {

     await FirebaseFirestore.instance
          .collection('userNotifications')
          .doc(currentSignedInUserID)
          .update({
        'messages': notificationsActive,
     });
    
    setState(() {
     notificationsActive = !notificationsActive;
    });
  }

And also I would suggest to you to listen to your Firestore collection and update UI on change. You can look up on how to do that here.