On demand rules
There are some firewalls that allow you to use rules on demand, which alert the user whenever a connection attempt is made. The first program in which I knew this function was the comodo firewall, but today there are great open source options, such as simplewall (windows) and OpenSnitch (linux).
Since the plasma-firewall is in the early stages of development this can be a great option to add, as it considerably improves the security and privacy of the system.
The OpenSnitch developer shows great examples here https://github.com/evilsocket/opensnitch/wiki/OpenSnitch-in-action