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

Thanks for pointing me to the troubleshoot information, however, I had already looked at the troubleshoot information prior to posting the question and it did not resolve my problem. I have to add that this is happening on two different systems, on both of which I could load this page not long ago, and that have had no software change or update recently. Moreover, I have to add I just noticed that in internet explorer the site also fails to load.

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

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

Start your Computer in safe mode with networking. Then start Firefox. Try Safe websites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Hi teac, it's possible that Mozilla uses a number of different geographically dispersed servers for that page. Here in California, I'm not noticing any problem.

Are the two different systems that have the problem within the same network, i.e., connecting to the internet through the same router?

Does the error page you get in Firefox or in Internet Explorer provide more information? For example, in Firefox, you may find an "Advanced" button to expand the description and/or provide certificate information when you click a link. This usually is helpful in getting to the root of problem. Anything like that?

Dear all, Thanks for all your suggestions.

Today when I tried to access the firefox site it was working normally again. Not sure what caused the problem, maybe it was a problem with the local server, maybe it was on my end. Anyway, I am happy it is all working again normally.

Keep an eye on it for a few days. If you think the problem is gone, Please flag your last post as Solved Problem so others will know.