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To block internet access to YouTube, add it to the main machine's firewall, or add a rule to block these websites to each specific computer if that is the way the network is set up.

Morning Percent.
Are you mean the 'main machine firewall' is inside the mozilla setting preferences? like window firewall that inside the  our staff pc control panel.
sorry, i dont get it.

I didn't know what operating system you were using, but it is now clear to me that you are using Windows. Do your computers connect via your WiFi network?