For existing configurations, it makes it a bit trickier to implement. If you ad the aforementioned rules to your DNS and/or IP block list, then Firefox will def

For existing configurations, it makes it a bit trickier to implement. If you ad the aforementioned rules to your DNS and/or IP block list, then Firefox will default back to using the system configured DNS. But DoH is not targeting ad-blocking specifically, but rather intermediaries that are outside of the local network. There is evidence of ISPs injecting traffic (Comcast/Xfinity) or selling user traffic (AT&T) and this was designed to close one of the last gaps for a fully encrypted flow. It's possible to setup a DoH server for your local network's DNS resolver, so that all of your traffic leaves your network encrypted, even if not encrypted on your local network.