Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

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

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

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

As I mentioned before, I tried safe mode and I still have the problem. I also don't have any of that software.

I was able to access a non https website this morning until the notification in the picture came up and then it has stopped again. This is a different app from last night, both of which I have removed. I have also cleared cookies and the cache.

What is "nitter" ?

Did you check the links I posted?

I don't know what it is. I think it was some kind of malware. I got rid of it and still have the problem.

None of the links helped sorry

Mac Malware Scanners https://discussions.apple.com/message/29938930#29938930

You can also try https://www.malwarebytes.org/antimalware/mac/index.html

The instructions in the apple discussion worked, apparently I had something that had to be removed manually. Thanks for your help!

Glad to help. Safe Surfing. Malware loves to hide deep in your system. Remember to run the scanners every now and then.