Mozilla Firefox Error Code SEC_ERROR_UNKNOWN_ISSUER: How to Fix It

Medium 10-20 minutes High Severity Verified June 2026
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Error Code
SEC_ERROR_UNKNOWN_ISSUER
Brand
Mozilla Firefox
Product Type
browser
Severity
High
DIY Difficulty
Medium
Estimated Fix Time
10-20 minutes
The SEC_ERROR_UNKNOWN_ISSUER error appears when Firefox cannot verify a website's security certificate because it was signed by an unrecognized Certificate Authority (CA). This commonly occurs with corporate proxies, self-signed certificates, or outdated security settings blocking legitimate websites.
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Tools You'll Need

How to Fix Error Code SEC_ERROR_UNKNOWN_ISSUER

  1. Check Your System Date and Time

  2. Disable Antivirus SSL/HTTPS Scanning

    Only disable temporarily for testing. Re-enable after confirming the fix.

  3. Clear Firefox Certificate Cache

    Do not delete well-known CAs like DigiCert, Let's Encrypt, or VeriSign as this will break other websites.

  4. Reset Firefox Security Settings

    These changes reduce security temporarily. Reset to default values after fixing the certificate issue.

  5. Add Certificate Exception (Temporary Fix)

    Only use this for trusted websites. Adding exceptions for unknown sites creates security risks.

  6. Import Corporate or Custom Certificates

    Only import certificates from trusted sources. Malicious certificates can compromise your security.

  7. Create New Firefox Profile

    Creating a new profile will reset all your settings, bookmarks, and extensions. Export bookmarks first if needed.
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When to Call a Professional

Contact your network administrator or IT support if you're on a corporate network and none of these solutions work. Professional help may be needed if the issue affects multiple browsers or appears to be related to network infrastructure, malware, or compromised system security settings.

Frequently Asked Questions

What causes SEC_ERROR_UNKNOWN_ISSUER in Firefox?
This error occurs when Firefox cannot verify a website's SSL certificate because it was signed by an unrecognized Certificate Authority. Common causes include corporate firewalls, antivirus HTTPS scanning, self-signed certificates, or incorrect system time.
Is it safe to add a certificate exception in Firefox?
Adding exceptions should only be done for websites you trust completely, such as internal corporate sites or development servers. Never add exceptions for unknown websites as this bypasses important security protections.
Why does this error only appear in Firefox and not other browsers?
Firefox maintains its own certificate store separate from the operating system, while Chrome and Edge use the system certificate store. This means Firefox may not recognize corporate or custom certificates that other browsers accept.
How do I permanently fix SEC_ERROR_UNKNOWN_ISSUER for corporate websites?
Contact your IT department to obtain the proper corporate CA certificate file. Import this certificate into Firefox's trusted authorities list through the Preferences > Privacy & Security > Certificates section.
Can antivirus software cause SEC_ERROR_UNKNOWN_ISSUER?
Yes, antivirus programs that scan HTTPS traffic often insert their own certificates, which Firefox may not recognize. Temporarily disabling SSL/HTTPS scanning in your antivirus settings can resolve this issue.

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