There is a special page in Firefox that shows information about Firefox's DNS settings, along with a button to 'Clear DNS Cache' which comes in handy when developing sites and making DNS changes. Otherwise Firefox can sometimes take a longtime to update stale DNS entries. Also can be helpful when editing /etc/hosts directives locally. Access the page by visiting
about:networking#dnsin the Firefox URL bar.
This page also shows the currently configured 'DNS Suffix' and 'DNS over HTTPS' endpoint, though it doesn't state here whether or not DNS over HTTPS is enabled or not. Good reminder to check your about:config network.trr.mode to 5:
5 - Off by choice. This is the same as 0 but marks it as done by choice and not done by default.
if you're using your own DoH setup, otherwise Firefox will ignore your system settings and route your DNS traffic to a Cloudflare DoH endpoint.