firewalld under at least Fedora, and probably CentOS, can be annoying because you can't disable it - it always comes back up on a reboot. So, the easier thing is to just leave it enabled and add the services/ports you want it to allow.
Here's how with firewall-cmd:
$ sudo firewall-cmd --zone FedoraServer --add-service=http
success
$ sudo firewall-cmd --zone FedoraServer --add-service=https
success
$ sudo firewall-cmd --zone FedoraServer --list-services
cockpit dhcpv6-client http https ssh
$ sudo firewall-cmd --runtime-to-permanent
success
$ sudo systemctl restart firewalld
The third command just lists the services on the FedoraServer zone so you can check that you indeed added http and https (or whatever). The fourth one copies the runtime config you just made to the permanent config. The fifth restarts firewalld with the new configuration, which will also be loaded on boot since you made it permanent.
If you just want to open a random TCP port (not the default one associated with a standard service) use:
$ sudo firewall-cmd --zone FedoraServer --add-port=12345/tcp
$ sudo firewall-cmd --zone FedoraServer --list-ports
12345/tcp
Or remove a port:
$ sudo firewall-cmd --zone FedoraServer --remove-port=12345/tcp
or service:
$ sudo firewall-cmd --zone FedoraServer --remove-service=https