What's your computer system and Firefox version?

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, Kaspersky, 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/firefox-and-other-browsers-cant-load-websites

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://www.bing.com/search?q=web+site+access+denied

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.

Thanks, that points m,e in the right direction.

I'm using the latest Firefox, 49.0.2. Apple 10.9.7 Ubuntu Mate, the latest, I'm not in front my laptop.

I'm assuming the blocking is Flash related?

I will try your suggestion, but won't it cripple Firefox?

33Nicolas said

I'm assuming the blocking is Flash related?

I will try your suggestion, but won't it cripple Firefox?

Flash is used as a player. Not for blocking.

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.

Yes, Flash is a display program. What I meant is the predominant use of Flash is certainly what then new security update is trying to block. Unfortunately, it's blocking a lot of sites with I assume flash buttons and more.

Unfortunately, I need to access them. That's the problem with the last Firefox updates. Many sites just won't work anymore.

You guys were right, this fixed it.

It wasn't a flash problem, but a Java Script.

Sigh...

Thanks for the help and how promptly you guys answered it. This rocks.

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.