Firefox Edge Browser Error Code SEC_ERROR_UNKNOWN_ISSUER: How to Fix It

Q: Is it safe to click 'Accept the Risk and Continue' on the SEC_ERROR_UNKNOWN_ISSUER warning?

Generally, no — bypassing this warning means you are proceeding to a site whose certificate Firefox cannot verify, which could expose you to man-in-the-middle attacks or a fake website. Only bypass the warning if you fully trust the site and understand why the certificate is untrusted (for example, a local development server or an internal company tool with a self-signed certificate).

Q: Why does SEC_ERROR_UNKNOWN_ISSUER appear in Firefox but not in Chrome or Edge?

Firefox uses its own built-in certificate store rather than relying solely on the operating system's certificate store. Chrome and Edge use the Windows or macOS system store, which may already trust the certificate in question. This means a certificate trusted by Windows may not automatically be trusted by Firefox, which is why antivirus root certificates or corporate CA certificates often need to be added to Firefox separately.

Q: Why did this error appear suddenly on a site that worked fine before?

Several things can trigger a sudden appearance of this error: the website's SSL certificate recently expired or was replaced with one from a less-trusted authority, your antivirus software updated and changed how it performs HTTPS scanning, your system date/time drifted and is now outside the certificate's validity window, or a Firefox update changed its certificate trust rules.

Q: How do I fix SEC_ERROR_UNKNOWN_ISSUER for an internal company website?

Internal company websites often use certificates signed by a private, internal Certificate Authority (CA) that Firefox does not know about by default. Ask your IT department for the internal root CA certificate file (.crt or .pem), then import it into Firefox via Settings > Privacy & Security > View Certificates > Authorities > Import. Check the box to 'Trust this CA to identify websites' and save. The error should disappear for all internal sites using that CA.

Q: Can a VPN cause the SEC_ERROR_UNKNOWN_ISSUER error in Firefox?

Yes. Some VPN clients or proxies perform their own SSL inspection and replace website certificates with ones signed by the VPN's own certificate authority. If that CA certificate is not in Firefox's trust store, you will see this error. Try disabling your VPN temporarily to test. If the error goes away, check your VPN application's settings or support documentation for instructions on installing its certificate into Firefox.

Abstract

The SEC_ERROR_UNKNOWN_ISSUER error in Firefox means the browser cannot verify the certificate authority (CA) that issued the SSL/TLS certificate for the website you are trying to visit. This typically happens when the certificate chain is broken, self-signed, intercepted by security software, or not trusted by Firefox's built-in certificate store. The error is rated medium severity because it blocks access to the site but can usually be resolved through software or settings adjustments without any hardware work.