What is the exact error message?

your connection is not secure. the owner of support.mozilla.org has onnfigured their web site improperly

further to my previous reply ,there is another error code, SEC_ERROR_UNKNOWN_ISSUER

That information is what we needed.

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

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

• uses an invalid security certificate SSL_ERROR_BAD_CERT_DOMAIN
• configured their website improperly

How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

What security software do you have (Avast, Kaspersky or other)?

AVG for mac (free version)

Hi,

AVG says it's a bug and they're working on it.

See the last post in this thread :

https://support.avg.com/answers?id=9060N000000U3wAQAS

hi, thanks for everyones replies, it would appear that the issue is caused by firefox itself, protecting, because of certificates that are sent by avg that it does not recognise, this is due to the way avg works, if you look back at previous posts, theres a really good explanation, SEC_ERROR_UNKNOWN_ISSUERi have fixed the problem by switching to AVAST

Before jumping out of the frying pan into the fire, would you please take a look at these articles (don't want you to get anymore problems) :

https://www.macworld.co.uk/how-to/mac-software/do-macs-get-viruses-do-macs-need-antivirus-software-3454926/

And :

https://www.macworld.co.uk/feature/mac-software/best-mac-antivirus-2018-3637103/

And of course don't forget that you're using the safest browser on the
planet  !