You can install ufw and a frontend for it that lets you block specific processes. https://wiki.archlinux.org/title/Uncomplicated_Firewall#GUI_frontends It seems KDE already comes with a frontend in the system settings, and there’s gufw for gnome/gtk.

I read you mentioned firefox. I had a similar experience a while ago, related to this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1704774#c13

What you should be asking is whether the cables qre the bottleneck in your network or not.

Is there any link that is not negotiating 1Gbps? Do you have devices that could push 10Gbps but the cable is not allowing it? If not, then there’s no need to upgrade them.

Unless, of course, if you want to do it just for fun, which is also a legitimate reason 😄