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When Firefox can’t connect to a website securely, you may see a Secure Connection Failed or Warning: Potential Security Risk Ahead message. This usually means:
- Firefox can’t verify the site’s security certificate, or
- Something on your computer or network is blocking or replacing the secure connection.
This article shows you how to find your error code, understand what it means, and fix it.
Table of Contents
Find your error code
Your error code tells you exactly what type of secure connection problem you have. When you see a page like this, Firefox has blocked the site because it detected a possible security problem.
- Click the Advanced… button on the warning page.
- Find the Error code text, similar to the following: `SEC_ERROR_…`, `SSL_ERROR_…`, or `MOZILLA_PKIX_ERROR_…`.
- Write down the code and match it to the table below.
| Error code | What it means | How to fix |
|---|---|---|
| SEC_ERROR_EXPIRED_CERTIFICATE | The site’s security certificate has expired. | Check your device’s date/time. If it’s correct, the issue is with the site’s certificate and you’ll need to contact the site owner. |
| SEC_ERROR_UNKNOWN_ISSUER | Firefox doesn’t trust the certificate issuer (often antivirus SSL scanning or corporate proxies). | Turn off antivirus SSL scanning or contact your IT department. |
| MOZILLA_PKIX_ERROR_MITM_DETECTED | Firefox detected a man-in-the-middle interception (antivirus, firewall, or proxy). | Disable HTTPS scanning in antivirus. Try another network. |
| SSL_ERROR_BAD_CERT_DOMAIN | The certificate is for a different site. | Check the address. If correct, the site owner must fix the certificate. |
| PR_END_OF_FILE_ERROR | The secure connection was closed unexpectedly. | Try disabling VPN/firewall temporarily or switching networks. |
Check if the website is down
If a website won’t load in Firefox, it might be a problem on the site’s end.
- Try opening the same site in another browser.
- If it doesn’t load there either, the website is likely experiencing issues.
Verify your computer’s date and time
An incorrect clock or time zone can make valid security certificates look expired or invalid.
Make sure your device’s date, time, and time zone are correct.
Temporarily disable antivirus SSL scanning
Some antivirus programs scan secure connections and replace the website’s certificate with their own, which can cause Firefox to block the site.
- Open your antivirus software.
- Look for an SSL or HTTPS scanning option.
- Turn it off, reload the site, and then turn it back on afterward.
Check firewall, proxy, or VPN settings
Corporate or school networks may block certain certificates.
- Try disconnecting from your VPN or switching networks.
- If you are on a managed network, contact your IT department.
- Try opening another trusted website. If that works, the problem is likely with the original site.
Clear cached certificates in Firefox
If Firefox has saved an outdated or untrusted certificate, removing it may fix the error.
- Click the Firefox menu and select Settings.
- Go to the Privacy & Security panel.
- Scroll down to the Certificates section and click View Certificates….
- Select any untrusted or outdated site certificates and click Delete or distrust….
