This is intentional. The firewall pane is read-only when “advanced user” mode is not enabled. The reason is to provide some awareness to non-advanced users about where their browser connects – hopefully the gained awareness will motivate some users to take action and start to control about whether they should block by default some or all 3rd-party connections.
See https://github.com/gorhill/uBlock/wiki/Quick-guide:-popup-user-interface#the-overview-panel.