When a service is enabled - the ports required to connect to it will become active.
A default Manjaro installation has no activated services which is - with the exeption of Openbox, LXDE and LXQt editions - why the firewall is not enabled.
Unless your system is exposed to the internet either directly or via routing using portforward there should be no issues.
If - on the other hand - you intend to make it accessible from outside your LAN you should implement the necessary security measures. Of course implementing security inside your LAN is also possible. See the following topic