MOZILLA_PKIX_ERROR_MITM_DETECTED in Firefox

• Corporate proxy with TLS inspection (Zscaler, Forcepoint, Symantec)
• Kaspersky Internet Security / Bitdefender with SSL inspection
• Router-level parental controls
• Actual MITM attack (public Wi-Fi, compromised DNS)
• Hand-rolled MITM test proxy (mitmproxy, Burp)

1. If in a corporate network — expected, contact IT
2. Temporarily disable antivirus SSL inspection — retest
3. Switch network (mobile hotspot) — if it's gone = your network
4. Firefox: about:preferences#privacy → Certificates → "Ask every time"
5. DO NOT click "Advanced → Accept risk" unless you understand the cause

Check SSL Certificate →

• NET::ERR_CERT_AUTHORITY_INVALID
• ERR_SSL_PROTOCOL_ERROR
• SSL Handshake Failed
• All SSL errors

The MOZILLA_PKIX_ERROR_MITM_DETECTED error in Firefox indicates that a man-in-the-middle (MITM) attack has been detected, often due to improper SSL certificate validation. To resolve this, ensure your connection is secure, verify the server's SSL certificate, and check for installed root certificates. If necessary, clear your browser cache and restart Firefox. For persistent issues, consider disabling any security software that may be interfering.

The MOZILLA_PKIX_ERROR_MITM_DETECTED error occurs when Firefox identifies a potential man-in-the-middle attack during an SSL/TLS handshake. This typically happens when the browser detects that the SSL certificate presented by the server does not match the expected certificate, often due to interception by proxies or firewalls. This security measure is part of Firefox's commitment to protecting users from potential data breaches.

Common causes include:

• Proxy Servers: If you're using a proxy server, it may be intercepting SSL traffic, leading to certificate mismatches.
• Antivirus Software: Some security software performs SSL scanning, which can cause Firefox to flag the connection.
• Outdated or Misconfigured Certificates: If a website's SSL certificate is outdated or improperly configured, it may trigger this error.

To further diagnose the issue, you can check the Firefox certificate manager by navigating to about:preferences#privacy and clicking on View Certificates. This allows you to inspect the certificates installed and those that are trusted.

Resolving the MOZILLA_PKIX_ERROR_MITM_DETECTED error may involve a series of troubleshooting steps. Below are detailed instructions to help you address this issue effectively.

1. Verify Your Network Configuration

Start by checking if a proxy or firewall is affecting your connection:

1. Open Firefox and navigate to about:preferences.
2. Scroll down to Network Settings and click on Settings....
3. Ensure that No proxy is selected unless you specifically need a proxy. If you must use a proxy, verify that it is correctly configured.

2. Check Your SSL Certificate

Next, verify the SSL certificate of the site you are trying to visit:

1. Click on the padlock icon in the address bar.
2. Select Connection Secure and then click on More Information.
3. In the Page Info window, click on View Certificate to check if the certificate is valid and issued by a trusted authority.

3. Disable SSL Scanning in Antivirus Software

If you suspect your antivirus software is causing the issue, you can temporarily disable SSL scanning:

1. Open your antivirus application.
2. Look for settings related to Web Protection or SSL Scanning.
3. Disable the SSL scanning feature and save your changes.

Remember to enable it again after troubleshooting to ensure continued protection.

4. Clear Browser Cache

Clearing your browser cache can also help resolve this issue:

1. Go to about:preferences#privacy.
2. Under Cookies and Site Data, click on Clear Data....
3. Select Cached Web Content and click Clear.

5. Restart Firefox

After making these changes, restart Firefox to apply the settings. If the error persists, consider checking for Firefox updates or resetting Firefox to its default settings.

6. Additional Troubleshooting

If the issue continues, you can check for specific Firefox errors in the console. Open the console with Ctrl + Shift + K and look for any relevant error messages that can provide further insights.