Hi, have you checked the Options page? this setting is a little hard to find. Either:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the search box at the top of the page, type network and Firefox should filter to the "Network Settings" section. Click the "Settings" button there to pop up an overlay with details. The "Enable DNS over HTTPS" option is toward the bottom of that panel.

Cloudflare Diagnostic Page

If Firefox turns on the default provider, Cloudflare, then when you go to the following address, Cloudflare will indicate that DNS over HTTPS is enabled:

https://1.1.1.1/help

I don't know whether NextDNS or other providers have a similar test page.

I have already opted for DNS over HTTPS in the options section. The FAQ page says:

In addition, Firefox will check for certain functions that might be affected if DoH is enabled, including: Are parental controls enabled?

  Is the default DNS server filtering potentially malicious content?
  Is the device managed by an organization that might have a special DNS configuration? 

If any of these tests determine that DoH might interfere with the function, DoH will not be enabled. These tests will run every time the device connects to a different network.

My question is, if Firefox checks for affected functions and then overrides my option and disables DNS over HTTPS is there a way to find out?

Went to 1.1.1.1 and it says 'Yes' for using DNS over HTTPS. So I understand that my option to use DOH has not been overridden. Thanks