How to Repair: Firefox MOZILLA_PKIX_ERROR_MITM_DETECTED Error Code

  • Mozilla Firefox users experience problems accessing various Web sites with an HTTPS prefix.
  • Firefox does not trust the integrity of the certificates issued for the websites you visit.
  • This usually happens when malware tries to replace a legitimate certificate with its own.
  • In this article, we will show you how to fix the “MOZILLA_PKIX_ERROR_MITM_DETECTED” problem.

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In the past few days, a had som trouble with my antivirussoftware. Support uninstalled AdGuard for Windows for some reason. Now AdGuard is back up and running, but Firefox cannot open a page. The error code is MOZILLA_PKIX_ERROR_MITM_DETECTED.

It is in Firefox 61 that Mozilla introduces a warning about man-in-the-middle SSL attacks, “MOZILLA_PKIX_ERROR_MITM_DETECTED”, which warns the user when an SSL attack is being attempted. Mozilla explained in Firefox 65 that this error can be caused by software, such as an antivirus program.

Despite its beta testing phase, Mozilla Firefox has been praised for its security, speed, and useful add-ons. Firefox remains one of the most popular browsers in the world. As with any browser, it has its problems. The following error messages are commonly received by Firefox users when trying to access Web sites with HTTPS prefixes.


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 “PKIX_ERROR_MITM_DETECTED in Mozilla” problem.

What causes PKIX_ERROR_MITM_DETECTED in Mozilla?

There may be a proxy server on the other end of the wire intercepting your connection and causing the MOZILLA_PKIX_ERROR_MITM_DETECTED error to appear.

A security issue with your network connection causes the MOZILLA_PKIX_ERROR_MITM_DETECTED Firefox browser error code. Fortunately, some solutions prove to be helpful when it comes to the problem. It is primarily a trust issue with Firefox as your browser cannot trust privacy certificates.

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.

How to solve the PKIX_ERROR_MITM_DETECTED in Mozilla error?

Updated: February 2024

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Disable VPN or proxy server

Disable VPN

  1. Click the Control Panel icon and select Programs and Features.
  2. Your VPN server can be located by using the following steps.
  3. Choose Uninstall from the menu when you right-click the name.
  4. The VPN can be uninstalled by following the instructions.
  5. Your computer needs to be rebooted.

Proxy server needs to be disabled

  1. By pressing both the Windows key and the R key, you can open the Windows Run dialog box.
  2. Enter ms-settings:network-proxy.
  3. Click on “Proxy”.
  4. You can find the Manual Proxy Setting option on the right side of the window.
  5. Disable the button by toggling it.
  6. Your computer needs to be restarted.

Remove the antivirus software

  1. Control panel should be open.
  2. In Programs and Features, click on the program you want.
  3. Click the right-click icon next to your antivirus software.
  4. Uninstall the program by selecting it.
  5. Your computer needs to be restarted.

In addition to making your system vulnerable to attacks, uninstalling antivirus software can make your system susceptible to various types of malware.

Disable security.enterprise_roots.enabled in Firefox settings

  1. Open Firefox.
  2. You must type about:config into the address bar. Hit Enter.
  3. Once you click OK or Yes, a confirmation message will appear.
  4. Security.enterprise_roots.enabled can be found here.
  5. The value can be changed by double-clicking it.
  6. You will need to restart Firefox.

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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.