Firefox Certificate Serial-Number Errors: Safe Troubleshooting

Firefox’s SEC_ERROR_REUSED_ISSUER_AND_SERIAL means a certificate conflicts with another certificate using the same issuer and serial number. It is not simply an expired certificate, and clearing unrelated trusted roots is not a safe general fix.

Identify the affected service

Record the exact hostname, browser error code and time. Do not enter a password through a failing connection. Check whether the problem concerns a website, a control panel or an appliance you administer; each can use a different certificate.

Have the administrator correct the certificate

The administrator should inspect the presented certificate and its issuer, then replace an incorrectly regenerated certificate with one having a unique serial number. Check that the replacement covers the hostname, has a valid chain and is installed on the actual endpoint, including a proxy or load balancer when applicable. A browser-side workaround does not correct the server’s certificate issuance.

Review a saved exception only after identifying it

Mozilla documents that older devices can regenerate certificates incorrectly. If you previously saved an exception for that exact device, follow Mozilla’s current certificate-manager instructions to inspect that specific saved entry. Verify the device and replacement certificate through a trusted channel before removing an exception and reconnecting. Do not delete an entire trusted-root store, a certificate for an unrelated hostname, or a certificate just because its label resembles the affected service.

If this affects an InterServer endpoint, open a support ticket with the hostname and error code. Do not attach passwords or private keys. The old Internet Explorer/Netscape instructions in this article have been retired; they do not describe current browser interfaces.

Verify the repair

Reconnect without bypassing warnings. Confirm the expected hostname and certificate details and that the serial-number error no longer occurs. If it persists, send the administrator the new error details rather than weakening certificate validation.

Reference: Mozilla’s certificate serial-number troubleshooting.

Reviewed October 14, 2026. Documentation review; no customer certificate store or server certificate was changed.