It is currently being rolled out for the US customers in a phased manner.
https://blog.mozilla.org/blog/2020/02/25/firefox-continues-push-to-bring-dns-over-https-by-default-for-us-users/
From the source link above,
Because there is no encryption, other devices along the way might collect (or even block or change) this data too. DNS lookups are sent to servers that can spy on your website browsing history without either informing you or publishing a policy about what they do with that information.
Mozilla claims that more browsers are thinking the same way and even Facebook is thinking about a secure DNS.
Mozilla says users outside the US can also use DoH feature. Here is how
If you’re outside of the US and would like to enable DoH, you’re welcome to do so by going to Settings, then General, then scroll down to Networking Settings and click the Settings button on the right. Here you can enable DNS over HTTPS by clicking, and a checkbox will appear. By default, this change will send your encrypted DNS requests to Cloudflare.
I don't know about the last part. So they are including a third party in the whole process ?
I will wait for more feedback before going with the setting. A particular blog which I saw was criticising the DOH move stating that it will block legitimate attempts by service to block dangerous websites.
