[Affected versions]:

• Firefox Release 90.0 (Build ID: 20210705185941)
• Firefox Beta 91.0b3 (Build ID: 20210715191625)
• Firefox Nightly 92.0a1 (Build ID: 20210716094806)

[Affected Platforms]:

• Windows 10 x64;
• macOS 10.15.7;
• Linux Ubuntu 20.04 x64;

[Steps to reproduce]:

1. Open the Firefox browser.
2. Navigate to the “about:preferences: page -> Network Settings section.
3. Click the Settings button.
4. Check the “Enable DNS over HTTPS” option and click the providers drop-down button.
5. Select a DNS provider, other than the default one (e.g. NextDNS).
6. Click the “OK” button and navigate to the “about:telemetry” page.
7. Search for the “network.dns” event and observe the TRR confirmation time value.

[Expected result]:

• The value of the TRR confirmation time is greater than 0.

[Actual result]:

• The value of the TRR confirmation time is 0.000000.

[Notes]:

• This issue is intermittent.
• Attached is a screenshot of the telemetry probe.

I think this happens whenever there's another confirmation in progress.
I think I should be be able to write a unit test for this.