Hello,
For installing of Network Threat Protection (Firewall) feature, you may need to follow the Auto-upgrade feature.
NOTE: A restart of machine is required after installing the Network Threat Protection (Firewall) feature
Check these Articles:
Upgrading clients by using AutoUpgrade
http://www.symantec.com/docs/HOWTO17911
Check this Article and Video below:
Upgrade clients to SEP 12.1 by Auto upgrade feature
https://www-secure.symantec.com/connect/articles/upgrade-clients-sep-121-auto-upgrade-feature
Upgrade clients to SEP 12.1 using Auto upgrade feature
https://www-secure.symantec.com/connect/videos/upgrade-clients-sep-121-using-auto-upgrade-feature
How to add or remove features to existing Symantec Endpoint Protection (SEP) client installations
http://www.symantec.com/docs/TECH90936
Hope that helps!!