Firefox have recently launched a feature that allows you to encrypt your DNS traffic, historically DNS uses a combination of UDP and/or TCP on port 53 which has always been available to see in plain text. This controversial feature moves DNS away from the network OS level to the application level.
With this setting enabled the domain name you typed is sent to a DNS-over-HTTPS (DoH) compatible server using an encrypted HTTPS connection instead of a plain text one. This prevents third parties (malicious or not) from observing your DNS traffic.
This article shows you how to enable the setting in Firefox and show the setting in action within Wireshark.