A simple solution would be to add the shell protocol to this list.
Personally I think a secure blacklist is hard to maintain as new
dangerous external protocols could be invented by third-parties leaving
Mozilla vulnerable again.
There should be a whitelist, not a blacklist... a safe protocols list.
And what would happen?
Nobody would configure anything but those.
And what would happen next?
People would find ways to put their "new stuff" inside the "safe ones".
Kind of like how "http" is declared safe (but is it really??) and so
every man and their dog tunnels their proprietary stuff through that
because it'll go through firewalls.