Look at the "Issued By" in your picture.

There is security software like Avast and Kaspersky and BitDefender and ESET that intercept secure connections and send their own certificate.

http://www.ehow.com/how_11385212_troubleshoot-reset-connection-firefox.html

https://support.mozilla.org/en-US/kb/server-not-found-connection-problem

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

http://kb.mozillazine.org/Error_loading_websites

This Connection is Untrusted is sometimes caused because the computer system clock is wrong. Check the time / date / time zone settings.

Your Kaspersky software should have updated Firefox automatically to trust its signing certificate. However, when that fails, you can manually import that signing certificate.

I found these steps in a post on the Kaspersky forums last Summer. The actual path on disk may vary depending on your product:

Open Firefox's Certificate Manager:

"3-bar" menu button (or Tools menu) > Options > Advanced > Certificates mini-tab > View Certificates button

In the Certificate Manager dialog, click the "Authorities" mini-tab (not the Servers mini-tab)

If you see an existing "Kaspersky Anti-Virus Personal Root Certificate"

Select it and Click "Delete or Distrust"

Now click "Import..."

Proceed to "C:\ProgramData\Kaspersky Lab\AVP15.0.1\Data\Cert\"

Select "(fake)Kaspersky Anti-Virus Personal Root Certificate.cer" and Open!

Does that work on your Firefox? If so, could you update the information, especially the folder on disk and name of the file (the last two steps)? Thanks.