Under Settings/Network/Smart Firewall/Advanced Settings, I have 'Automatic Program Control' set to Off. One of the main reasons I purchase NIS is for this feature.
When I upgraded to version 20 (from 19), whenever the firewall dialog appeared, it would cause Norton to crash (hang). Version 19 did not have this issue. I cannot be sure, but I suspect it has to do with multiple instances of this dialog appearing at the same time, such as at startup. (likely due to a combination with a large number of definitions under Settings/Network/Smart Firewall/Program Control; probably ~1000-2000 or so)
I uninstalled (deleting all user preferences), and reinstalled, and the issue has been slightly mitigated, but NIS has crashed one additional time. But, on the latest restart, things seem better. No NIS hangs on the firewall dialog on startup.