The latest ZoneAlarm® Free Firewall release is 14.2.255.000 For changes in this version, FREE ZoneAlarm ZoneAlarm® Extreme Security History ZoneAlarm with Antivirus ZoneAlarm Pro ZoneAlarm Security Suite ZoneAlarm® Free Antivirus + Firewall
Can't find anything current, but here's something from 2012: https://www.zonealarm.com/forums/showthread.php/78803-ZoneAlarm-Antivirus-Firewall-Free-vs-Pro Fax is a member here and can probably say more.
Long time not using the free version, so this may have changed. If not, the free version of the firewall has only basic feature for the programs control (hips) and you cannot open ports (i.e. create custom firewall rules).