How to fix the `MOZILLA_PKIX_ERROR_KEY_PINNING_FAILURE` error

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This article covers AdGuard for Windows, a multifunctional ad blocker that protects your device at the system level. To see how it works, download the AdGuard app

Some users of AdGuard for Windows with enabled HTTPS filtering are unable to access certain websites like google.com and youtube.com. Instead of opening a website, browsers with Strict Certificate Pinning would display a MOZILLA_PKIX_ERROR_KEY_PINNING_FAILURE error.

Here is a list of impacted browsers:

Arkenfox
Brace
Cachy Browser
Dove
FireDragon
LibreWolf
Mull
Mullvad Browser
Phoenix
Tor Browser

If your default browser is one of these, do the following to solve the problem:

Type about:config in the address bar of your browser
In the menu that appears, enter security.cert_pinning.enforcement_level in the search field.
Change the security.cert_pinning.enforcement_level parameter from 2 to 1.