To enable DoH, click on the three horizontal bars in the top right corner of Firefox and then select the "Options" button. (On MacOS, click Settings.)
Find the "Network Settings" heading, and then click the "Settings" button.
Scroll down Click "Enable DNS over HTTPS" and check or uncheck the box to turn the setting on or off.
If you enable DNS over HTTPS in this way, Firefox defaults to Cloudflare DNS. You can change your DNS by clicking the "Use Provider" drop-down menu and either clicking "NextDNS" to use the alternative selected by Mozilla, or "Custom" to manually enter the DNS server address you want. However, DNS over HTTPS only works if the selected DNS server has activated the support for it. You can't just use an old DNS server.
Even if you do not manually activate DoH, Firefox will automatically activate it for you in the next few weeks – provided you are in the United States. We assume that Mozilla will automatically activate DoH for people in other countries in the future.