Hi jaxx, no, there's no setting to restrict an add-on from accessing the internet. You shouldn't install extensions you can't trust to access the internet.

There should be a way to know the IP and port of the add-on and then block it in firewall.. But I don't know how to find the IP and port of the add-on. (if there is one - depending on the add-on) perhaps wire shark?

https://www.wireshark.org/download.html

Requests made by extensions would, I assume, be sent by Firefox. If a new local port is opened for the request, I don't know how you would learn of it. There is an internal page called

about:networking

but it only seems to show the remote port and not the local port.

Some extensions use external components that you need to install separately. Those might connect through another channel rather than through Firefox.