For those who are a bit confused by this and what this means, ultimately the switch to DNS over HTTPS is designed to help make your browsing experience more private and more secure. This is because based on the way the internet has been designed, while websites themselves might be encrypted, the actual lookup for the domain name isn’t. This means that your ISP will know the websites that you are visiting.
However by switching to DNS over HTTPS, your browsing sessions on Firefox will become even more private because even the domain name lookups will be encrypted. This means that even your ISPs have no idea which websites you are visiting. While not necessarily the best way of protecting your privacy online, by enabling it by default, it will help users who might otherwise be clueless about these things.
Mozilla says that the new feature will be rolled out in phases to users in the US, so if you don’t see it yet, not to worry as it should eventually find its way to you.