certificate-is-not-trusted-because-the-issuer-certificate-is-unknown

In Mozila FF I get this error:

https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean#w_the-certificate-is-not-trusted-because-the-issuer-certificate-is-unknown

Where in recent Avast Free Antivirus Do I forbid this?