If you don't know what it means or how it might affect you then please have a good read.
https://blog.mozilla.org/blog/2020/02/25/firefox-continues-push-to-bring-dns-over-https-by-default-for-us-users/If you are a private individual out and about roaming it may be a good thing.
If you run a network it may not.... but Uncle Mozilla has decided it know what is best for you, at least in the US.
I presume they are getting some sort of revenue kickback from Cloudflare somehow. People don't do these things for free.
While we would like to encourage everyone to use DoH, we also recognize that there are a few circumstances in which DoH can be undesirable, namely:
Networks that have implemented some sort of filtering via the default DNS resolver. This can be used to implement parental controls or to block access to malicious websites.
So DoH will bypass your PiHole or other network domain filtering. Both for good sites, and bad. Looks like those ads you have been blocking are going to work again !
How to disable:
https://support.mozilla.org/en-US/kb/configuring-networks-disable-dns-over-httpsIf you run a network you probably want to do it across your network.
There is a probably a way to block it with SME but I am not sure how - it needs to fail a lookup for "use-application-dns.net"
If you run a PiHole or similar you can blacklist the canary domain use-application-dns.net
Enjoy.