Hi,

Would you please take a look at this article :

https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

Does that help  ?

What's your computer system and Firefox version?

What is the full error message?

You can check if you have an "Old Firefox Data" folder on the desktop in case the Refresh feature in Firefox has been used and a new profile has been created.

• https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems

If that is the case then try to copy the cert8.db file from that folder to the current Firefox profile folder.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

user456 said

fix your browser. don't blame the user. blame your developers. certificate error while trying to access mozilla.org. apparantly mozilla is not a trusted site.

A few common reasons for getting certificate errors on sites. A antivirus pr parental software inserting its own certificate it its attempt at keeping you safe. The proxy is in use causing this. A longtime common cause is the system date/time is wrong. It is Windows users that encounter this more though 2+3 can be encountered by Mac and Linux users.