Firefox error code sec_error_unknown_issuer

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.

Staff Asked on October 3, 2017 in Firefox.
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3 Solution(s)

Solution 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.

Step 1. Disable/ Remove Extension/ Add-on

  1. Open Mozilla Firefox.

    RE: Firefox error code sec_error_unknown_issuer

  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.

Staff Answered on February 13, 2018.
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Solution 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.

RE: Firefox error code sec_error_unknown_issuer

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

Staff Answered on February 13, 2018.
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Solution 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.

Step 1.

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

    RE: Firefox error code sec_error_unknown_issuer

  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.

    RE: Firefox error code sec_error_unknown_issuer

  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.
Staff Answered on February 13, 2018.
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