You can enable or disable DoH in your Firefox connection settings:

1. Click the menu button and select Options.
2. In the General panel, scroll down to Network Settings and click the Settings… button.
3. In the dialog box that opens, scroll down to Enable DNS over HTTPS.

   • On: Select the Enable DNS over HTTPS checkbox. Select a provider or set up a custom provider.
   • Off: Deselect the Enable DNS over HTTPS checkbox.

   

4. Click OK to save your changes and close the window.

Source:

https://support.mozilla.org/en-US/kb/firefox-dns-over-https