One additional bit of information. I have multiple users set up on my mac and I am not getting the same errors with firefox when logged in under the other users - only my profile.

Clear the cache and remove cookies only from websites that cause problems.

"Clear the Cache":

• Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• Click the link at the bottom of the error page: "I Understand the Risks"
• Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate"
• Click the "View" button and inspect the certificate and check who is the issuer.

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

• https://support.mozilla.org/kb/Secure+Connection+Failed

Unfortunately none of those suggestions worked.

Found the problem. It has to do with Avast web shield.

Thanks for your help

Thanks for reporting that the problem was due to Avast web shield. I'll mark your question solved.

For others having this issue I found the following:

https://www.reddit.com/r/avast/comments/2t7n4k/untrusted_connections_after_updating_avast/

If you're getting an "untrusted connection" warning from Firefox when visiting Google and Youtube after recently updating Avast (or if you now can't log into reddit):

open Avast
Settings
Active Protection
Web Shield > Customize
untick Enable HTTPS scanning

Restart Firefox and the problem should be fixed.

P.S. For more information about Avast and HTTPS scanning, contact Avast support or see: https://forum.avast.com/index.php?topic=159346.0 Topic: Issues and questions on HTTPS Scanning - Avast Internet 2015