We need to disable add-ons because of VPN usage at our schools. We have successfully blocked chrome store with google by using our content filtering system by filtering out Chrome store URL. But, we are using Firefox more and more these days. Is there a URL that can be filtered to prevent any usage of Firefox add-ons? Or can add-on usage be disabled via GPO?

I do not like having to disable add-ons because there are way too many add-ons that can be used for educational purposes, but I am required to block students from accessing certain web sites. They are accessing the sites by installing VPN add-ons which circumvent out content filtering system.