Fix : MOZILLA_PKIX_ERROR_MITM_DETECTED issue in Mozilla Firefox

Mozilla Firefox has become quite known to users as the 4th most used browser in the world. While it keeps surprising us with new user-friendly features, aesthetics overhauls, well sometimes users are surprised by some annoying error messages. Like – “MOZILLA_PKIX_ERROR_MITM_DETECTED“. So, what to do if the browser refuses to connect to a particular website showing this error code in Advanced analysis? Don’t worry. There are quick and easy resolutions that can take care of this issue very quickly. Just follow these solutions.


Fix 1 – Turn on the HTTPS filtering

The issue seems to be happening due to the antivirus you are using. The antivirus may be acting as the MITM (Man in the Middle) and that is the reason Mozilla is refusing to connect. Enabling the HTTPS filtering request should fix the issue.

We have shown how to enable the settings in ESET antivirus.

1. Open ESET antivirus on your system.

2. The, tap on “Setup” from the left-hand side.

3. Next, click on “Internet Protection” to access it.


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4. After that, tap on the gear-shaped icon beside the “Email client protection” settings.


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5. Next, tap on the “WEB AND EMAIL” settings on the left-hand side.

6. Then, expand the “SSL/TLS” section.


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7. After that, toggle the “Enable SSL/TLS protocol filtering” to “ON” settings.

8. Finally, tap on “OK” to save the changes.


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Now, close the antivirus settings and reboot your machine. Open Mozilla Firefox and test whether this actually works out.


If you are using any other antivirus, follow these steps.

1. Open your antivirus settings.

2. Once the antivirus opens up, open the Web client settings.

3. You may find these different settings, depending upon the antivirus you are using –

Scan SSL
Enable SSL / TLS filtering
Enable HTTPS scanning
Show Safe Results Only


Just enable the setting you may find on your antivirus. Then, close it.

Finally, restart the computer. After that, open Mozilla Firefox and test whether you are still facing the error code or not.


Fix 2 – Switch off the HTTPS certificate check


Switching off the HTTPS certificate check will let you through to all the websites without showing any warning message. It can leave your system open to potentially harmful files that may appear from surfing unsafe pages. So, after fixing this issue, we recommend you enable it back again.

1. At first, open Mozilla Firefox on your system.

2. When the browser opens up, type this code in the browser and hit Enter.



You will notice that a warning message has appeared on your screen.

3. Then, tap on the “Accept the Risk and Continue“.


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4. Now, type “security.enterprise_roots.enabled” in the search box and hit Enter.

5. Then, tap on the two-side arrow icon () once to set the modifier to the “true” setting.


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Once you are done, close the browser. Then, relaunch it again. After opening Mozilla Firefox, try to access the problematic website once again.


If you feel that you have to revert this change to make the browser safe again, you can easily do it following these steps.

1. Open Mozilla Firefox.

2. Then, again type “about:config” and press the Enter key.

3. Tap on the “Accept Risk and continue” to proceed.


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4. Then, search for “security.enterprise_roots.enabled“.

5. After that, tap on the “Reset” button to reset the modifier to the default settings.


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Close and relaunch the Mozilla Firefox. Test whether this works out for you or not.


Fix 3 – Disable VPN/proxy

If you are using a VPN or proxy settings, you may face this issue.

1. At first, open the Settings.

2. Then, on the left-hand side, tap on the “Network & Internet” settings.

3. Next, on the right-hand side, click on the “VPN“.


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4. After that, on the right-hand side, you will find the VPN connection.

5. Now, tap on the drop-down beside the VPN settings that you are currently using and tap on “Disconnect“.

If you want, you can easily remove the VPN from your system. To do that, tap on “Remove“.


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After that, close the Settings and open firefox again. Check whether you are still facing the issue or not.

Fix 4 – Uninstall Legendas and stop Com+Leg Service

There are some users who have complained about the Legendas app is responsible for this issue.


1. At first, open the Settings window.

2. Then, tap on the “Apps” to access it.

3. After that, tap on “Apps and Features” to open it.


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4. Now, locate the “Legendas” app in the list. Just, tap on the three-dot menu and tap on “Uninstall” to remove it from your system.

Once you are done, you can close the Settings.


Step 2

You also have to disable the COM+Leg service.

1. Now, press Windows key+R keys together.

2. Then, type “services.msc” and click on “OK“.


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3. When the Services opens up, go to the “COM+ Leg Service“.

4. Then, right-tap on the service and tap on “Properties“.


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5. Next, toggle the ‘Startup type:’ to “Disabled“.

6. Finally, tap on the “Apply” and “OK“.


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Then, close the Services window. Restart your system once. After restarting the system,


Fix 5 – Disabling the antivirus

Test simply by disabling the antivirus temporarily.

1. At first, tap on the arrow icon on the taskbar.

2. Then, right-click on your antivirus and tap on “Pause protection” to pause the antivirus on your system.


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After that, open FireFox and check.


Fix 6 – Uninstall the third-party antivirus

If only disabling the antivirus doesn’t work out, uninstalling it from your system may help you resolve the issue once and for all.

1. Make sure that you have closed the antivirus completely on your system.

2. At first, press the Windows key+X keys together.

3. Then, tap on the “Apps and Features“.


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4. Next, search for your antivirus software in the list of apps.

5. Just, tap on the three-dot menu and tap on “Uninstall” to uninstall the antivirus from your computer.

You may have to confirm this with a click on the “Uninstall” prompt.


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Once you are done, close the Settings. Then, restart your system once. Check if this works out for you.

Some more Tips – 

1.  This can be a simple bug in your Firefox itself. Try to update your Mozilla FireFox.

a. Open Firefox.

b. Just tap on the three-bar menu and tap on “Help>“.


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c. Then, tap on “About Firefox“.


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Firefox will automatically search for an update and install it. You may have to relaunch the browser once.


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Your problem should be solved.