Today, Firefox began the rollout of encrypted DNS over HTTPS (DoH) by default for US-based users. The rollout will continue over the next few weeks to confirm no major issues are discovered as this new protocol is enabled for Firefox’s US-based users..
Outside the US? you can switch on DNS over HTTPS manually.
Firefox > Preferences
Scroll down to Settings
Click the Settings… button
At the bottom, check Enable DNS over HTTPS
Optionally, use the pull-down menu to change the provider