you are also me silly

Check this if it helps. https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

See also:

• https://support.mozilla.org/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

What security software do you have?

You can inspect the certificate and check details like the issuer of the certificate and other details like the expiration date.

• click "Advanced" to expand the error message
• click "Add Exception" to open "Add Security Exception"

If this isn't possible then open "Add Security Exception" by pasting this URL in the location/address bar and paste the URL of the website (https://xxx.xxx) in it's location field.

• chrome://pippki/content/exceptionDialog.xul

Let Firefox retrieve the certificate -> "Get Certificate"

• click the "View" button and inspect the certificate
  check who is the issuer of the certificate

You can see details like intermediate certificates that are used in the Delete tab.

Who is the issuer of the certificate?

This is not related, but on your plugin list, you have a really old version of the Adobe Reader plugin (vintage 2013). I suggest either updating Adobe Reader or disabling that plugin so you can avoid the security vulnerability of an out-of-date version.

To update Reader to version 11, you can use this page and lie to the site and tell it you have Windows 7 (or if you prefer, you can tell it you have Vista and download Reader 10, but that version is also "end of life" and not getting any more security updates).

https://get.adobe.com/reader/enterprise/

Either way, after installing, open Adobe Reader and go to its Help menu and Check for Updates to get the latest patches.

Alternately, if you don't like the Adobe product, you can disable the old plugin on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, find Adobe Acrobat and change its permission to "Never Activate" to hide it from websites.