How to Repair: Firefox MOZILLA_PKIX_ERROR_MITM_DETECTED Error Code

Mozilla Firefox remains one of the most popular web browsers in the world. Even in its beta testing phase, it was praised for its security, speed, and effective add-ons. But like other browsers, it is prone to problems. For example, many Firefox users experience problems accessing various Web sites, especially those with an HTTPS prefix. Most of them get the following error messages:


These error messages indicate that your web browser does not trust the integrity of the certificates issued for the websites you visit. The good news is that there are still ways to access these sites. In this article, we will show you how to fix the “MOZILLA_PKIX_ERROR_MITM_DETECTED” problem.


This means that something on your system or network is intercepting your connection and entering your certificates. When this happens, Firefox does not trust the certificate. This usually happens when malware tries to replace a legitimate certificate with its own. Surprisingly, this also happens with security software. They monitor a secure connection and give a false positive. Similarly, there may be a monitoring/filtering product on your corporate network that can replace certificates.


Disable VPN or proxy server

Disable VPN

  1. Go to Programs and Features in the Control Panel.
  2. Locate the VPN server you are using.
  3. Right-click the name and select Uninstall.
  4. Follow the instructions to uninstall the VPN.
  5. Reboot your computer.

Disable the proxy server

  1. Run the Windows Run dialog box. You can press the Windows key and the R key at the same time.
  2. Type ms-settings:network-proxy and press Enter.
  3. Go to the “Proxy” section.
  4. On the right side of the window, locate the Manual Proxy Setting option.
  5. Toggle the button to the Disable position.
  6. Restart your computer.

Remove the antivirus software

  1. Open the control panel.
  2. Go to Programs and Features.
  3. Find your antivirus software and right-click it.
  4. Select Uninstall.
  5. Restart your computer.

Note that uninstalling antivirus software will make your system vulnerable to various malware and other attacks.

Disable security.enterprise_roots.enabled in Firefox settings

  1. Launch the Firefox browser.
  2. Type about: config in the address bar. Press the Enter key.
  3. You will see a confirmation message. Click OK or Yes.
  4. Find security.enterprise_roots.enabled.
  5. Double-click it to change the value to True.
  6. Restart the Firefox browser.

Frequently Asked Questions

An incorrect system date can cause Firefox to detect that the site's security certificate is expired or invalid. Make sure your computer has the correct date, time, and time zone. For more information, see Troubleshooting time-related errors on secure websites.

If you click on the "Warning: Potential Security Risk Ahead" error page and see error code SEC_ERROR_UNKNOWN_ISSUER or MOZILLA_PKIX_ERROR_MITM_DETECTED, it means that the certificate provided was issued by a certificate authority that Firefox does not know and therefore cannot trust.

  1. On the error page, click Where you can add an exception.
  2. Click Add exception.
  3. Click Retrieve Certificate.
  4. Click View to display a window describing your z/OSMF site problem.
  5. After verifying the certificate, close the dialog box.
  6. Click Confirm Security Exception to trust the z/OSMF site.

Sites that are not encrypted are considered insecure by Firefox, and you will get this warning. In many cases, this warning is caused by the fact that the website is not properly configured. However, you can check the date and time and clear Firefox history to make sure it is not you.

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Mark Ginter is a tech blogger with a passion for all things gadgets and gizmos. A self-proclaimed "geek", Mark has been blogging about technology for over 15 years. His blog,, covers a wide range of topics including new product releases, industry news, and tips and tricks for getting the most out of your devices. If you're looking for someone who can keep you up-to-date with all the latest tech news and developments, then be sure to follow him over at Microsoft.