Firefox is planning to enable DNS over HTTPS soon. This will prevent services like DNS Filter from working, as well as affecting corporate split-DNS environments. (https://support.mozilla.org/en-US/kb/firefox-dns-over-https)
Mozilla provides a way to disable this feature using a DNS query, however it is so convoluted to implement that it's effectively not possible without major changes to internal DNS servers.
It is not possible to implement on older (but still supported) Windows DNS servers. (https://support.mozilla.org/en-US/kb/configuring-networks-disable-dns-over-https)
My suggestion is for DNS Filter to provide a service that allows customers to forward requests to the use-application-dns.net domain to receive the NXDOMAIN result, which would make it easy to apply this setting on any network.