I Don't know if this is it but Firefox have been advertising that they are going to make their DNS lookups SSL protected cloud based queries.
https://www.theregister.co.uk/2019/09/09/mozilla_firefox_dns/Check to see if this is turned on:
Enabling and disabling DNS-over-HTTPS
You can enable or disable DoH in your Firefox connection settings:
Click the menu button New Fx Menu and choose Options.
In the General panel, scroll down to Network Settings and click the Settings… button.
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.
Note: for a home user this is probably mostly ok
On a corporate computer many things may break and it should not be used. Particularly of duel brain DNS is in use as internal DNS searches will not be possible.