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Firefox error code sec_error_unknown_issuer

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By Genesis and SheilaUpdated on June 4th, 2022

What is 'Firefox error code sec_error_unknown_issuer'?

"Firefox error code sec_error_unknown_issuer" is an error that may occur when the certificate provided is issued by a certificate authority that is not known by Firefox and cannot be trusted by default.

How can you fix 'Firefox error code sec_error_unknown_issuer'?

Some of the common fixes include:

  • Disabling extensions and third-party programs
  • Scanning for malicious software
  • Disabling the network proxy

Please see our in-depth walk-through solutions below.

1. Disable Extension and Third-party program

"Firefox error code sec_error_unknown_issuer" may appear when your communication with the website is intercepted or injected by unknown data caused by a third-party software. This could be caused by a security software, browser extension/ add-on or malicious software. To fix the problem, you may want to try disabling all running third-party programs by booting your operating system in clean boot and removing or disabling all Mozilla Firefox extensions/ add-ons. See steps below.

Disable Extension and Third-party program
Image Credit: genesismarasigan / ErrorSolutions.tech

Step 1. Disable/ Remove Extension/ Add-on

  1. Open Mozilla Firefox.

  2. Click Shift+CTRL + A keys, to open about:addons page.

  3. Click the Extensions.

  4. Disable or Remove all Extension/ Add-on.

Step 2. Clean boot (watch the video below)


If the issue persists, try to scan your computer with an updated anti-virus.

2. Scan for malicious software

It is possible that the cause of “Firefox error code sec_error_unknown_issuer” is a malicious software, injecting an unknown data to steal information from you. To confirm, run an updated anti-virus and do a full scan of your system. If you do not have any anti-virus, check this link here to aid you to decide what legit antivirus software best work for you. If the antivirus is ready, check for an update before doing a full scan of your system.

Scan for malicious software
Image Credit: genesismarasigan / ErrorSolutions.tech

It may take a while. After the scan, restart your computer then try to browse again using Mozilla Firefox.

3. Disable Network Proxy

“Firefox error code sec_error_unknown_issuer” could also be caused by your proxy setting, Try to set it to default settings to fix the problem. See instructions below.

Disable Network Proxy
Image Credit: genesismarasigan / ErrorSolutions.tech

Step 1.

  1. Open Mozilla Firefox.

  2. In the address bar type about:preferences#general and press Enter key.

  3. Scroll down look for Network Proxy and press Settings.

  4. InConfigure Proxies to Access the Internet, select Use system proxy settings or No proxy.

Step 2.
Watch the instruction here on YouTube or refer to the image below.

  1. Click Windows Start, type in Internet Option and then click Enter key.

  2. Click the Connections tab and then click LAN settings.

  3. Then, check Automatically detect settings, and then uncheck Use a proxy for your LAN.

  4. Click OK then OK again.

  5. Run Mozilla Firefox again.

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