SSL certificates are now all but mandatory for any website. If you don’t encrypt your site, browsers will flag it as not secure. Moreover, your web pages won’t feature in search engine results pages. Whether you have just a blog or a large e-commerce platform, a valid SSL certificate is a must for your online success.

Unfortunately, many users encounter SSL connection errors after they install the SSL certificate. Most of the time, the issue is on the server-side, and the culprit is improper configuration. In some situations, the certificate itself is not to blame, and the problem lies on users’ computers.

Regardless of what triggers SSL errors, browsers display different warnings when they can’t connect securely to the webserver. In Firefox, a common issue is the SEC_ERROR_UNKNOW_ISSUER.
Its other variants are MOZILLA_PKIX_ERROR_MITM_DETECTED” or “ERROR_SELF_SIGNED_CERT”. It appears when the browser can’t verify the identity of the certificate issuer.

Date Invalid Firefox

In this tutorial, we will show you how to fix this problem to get your website back on track. If you see this website as a user, stay tuned as we have a few quick solutions for you too.

When this security warning pops up on a specific site, but others work just fine, the culprit is an invalid or faulty SSL certificate. As a user, you can’t do much about it, as only the website owner or administrator can access the server and rectify the error. If you see this message code on your website, follow the steps below:

Here are the main reasons that cause the Fix SEC_ERROR_UNKNOWN_ISSUER problem:

Missing Intermediate Certificate

A common mistake during SSL installation is a missing intermediate certificate. Intermediate certs are part of the SSL chain of trust and offer better compatibility on older devices and browser versions. Some CAs send you all SSL certs in a single file, while others put the root, intermediate, and server certificates in separate files.

With so many files and formats, inexperienced users get confused and overwhelmed during the installation process, which inevitably leads to SSL errors. To ensure your server has sent the corresponding intermediate certificates, follow a step-by-step installation tutorial and upload the certificates in the correct order.

Self-Signed SSL certificate

If you use a self-signed cert on your site, Firefox will display the ERROR_SELF_SIGNED_CERT error. Not a single browser will ever trust a self-signed certificate because anyone can issue it. Unless you deploy them in a testing environment, self-signed certs won’t do any good to your website. To get rid of this error, install a valid SSL certificate from a trustworthy Certificate Authority.


If your SSL configuration has loopholes, malware scripts or programs can intercept encrypted traffic and trigger similar security warnings in Firefox. To solve the malware issue, check this guide.

Corporate networks filtering

Corporate networks use different products to tighten their security. However, in some instances, traffic monitoring and filtering programs can replace the website certificate with their own, causing SSL errors. If you suspect this is your issue, ask your IT department to adjust Firefox settings for such an environment. You may need to add a root SSL to Firefox. Consult this Firefox wiki page for more details.

Antivirus programs

Third-party antivirus programs may conflict with secure Firefox connection. To rule this reason out, disable your antivirus for a while and restart the website. If the error persists, you’ve found the culprit. To avoid it in the future, adjust your antivirus settings, so it doesn’t block websites without your consent, or use the Microsoft Windows Defender instead.

If multiple sites display a Firefox error, and disabling your Antivirus doesn’t help, something on your system or network may intercept the connection and add certificates that Firefox doesn’t trust. In such an instance, you’ll see the MOZILLA_PKIX_ERROR_MITM_DETECTED error code. To fix it, you’ll have to manually delete the Firefox security certificate file called cert9.db. Here’s how to do it:

  • Open the Firefox settings by clicking the three horizontal lines at the upper right corner
  • Click Help than More troubleshooting information
  • Click on Information to solve the problem
  • Scroll down to the Profile Folder and click Open Folder
  • Now, go to Settings again click Exit
  • In the folder you’ve just opened, locate the cert9.db file and delete it.
  • Restart Firefox

Firefox Settings

Final thoughts

We’ve given you several solutions to tackle the most common Firefox connection errors, both as a webmaster and as a visitor. If you find any inaccuracies, or you have details to add to this SSL tutorial, please feel free to send us your feedback at [email protected] Your input would be greatly appreciated! Thank you.

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