Firefox SEC_ERROR_REUSED_ISSUER_AND_SERIAL — Certificate Error

Firefox SEC_ERROR_REUSED_ISSUER_AND_SERIAL error means a certificate with the same issuer and serial number already exists in the certificate store. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: Certificate serial numbers must be unique per issuer. Common in development environments using self-signed certificates. Firefox caches certificates more aggressively than Chrome. Regenerating a self-signed cert with the same serial triggers this. Clearing the cert database resolves the conflict. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Self-signed certificate regenerated with the same serial number. Development server certificate replaced without changing the serial. Corporate proxy certificate reissued with duplicate serial. Firefox certificate store has stale cached certificates. Multiple services using certificates from the same private CA with serial collision. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Delete the Firefox cert database: close Firefox, delete cert9.db from your Firefox profile folder. Profile location: about:support > Profile Folder > Open Folder. Alternatively: Preferences > Privacy & Security > View Certificates > delete the conflicting cert from Authorities tab. When regenerating self-signed certs, always increment the serial: openssl x509 -set_serial 0x$(openssl rand -hex 8). Restart Firefox after certificate changes. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

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