DNS over HTTPS (DoH) is a way to encrypt DNS queries and responses between a user's device and the resolving DNS server. For more on this see New in Simple DNS Plus v. 9.0.
To enable this in Firefox, do the following:
- Click the "hamburger" button (3 horizontal lines) in the top right corner of Firefox.
- Select "Settings...".
- Click "General" in the right side panel.
- Scroll down to "Network Settings" (at the bottom) and click the "Settings" button.
- In the "Network Settings" dialog, scroll to the bottom and check "Enable DNS over HTTPS".
- Here you can specify your DNS provider / enter a DoH query URL: