Sorry for the inconvenience. Usually if you get certificate errors on most secure sites, the problem is one of the following:

(1) Error in your system's date, time, or time zone, which throws off certificate validity checks. Sometimes allowing computers to use an internet-based time source can introduce this problem.

(2) Firefox not being set up to work with your security software that intercepts and filters secure connections. Products with this feature include Avast, BitDefender, Bullguard, ESET, and Kaspersky; AVG LinkScanner / SurfShield can cause this error on search sites.

(3) On Windows 10, Firefox not being set up to work with the parental control software Microsoft Family Safety. (To test by turning it off, see: http://windows.microsoft.com/en-us/wi.../turn-off-microsoft-family-settings)

(4) Malware on your system intercepting secure connections.

If you have any of those specific security products:

That would be the first thing to check. This support article will walk you through it: How to troubleshoot security error codes on secure websites.

If none of those ring a bell:

You could inspect a sample certificate to see whether that points to the culprit. If you want to try that now, here's how I suggest starting:

Load my test page at: https://jeffersonscher.com/res/jstest.php

You likely will get an error page. Expand the "Advanced" button and look for an Add Exception button.

Note: You don't need to complete the process of adding an exception -- I suggest not adding one until we know this isn't a malware issue -- but you can use the dialog to view the information that makes Firefox suspicious.

Click Add Exception, and the certificate exception dialog should open.

Click the View button. If View is not enabled, try the Get Certificate button first.

This should pop up the Certificate Viewer. Look at the "Issued by" section, and on the Details tab, the Certificate Hierarchy. What do you see there? I have attached a screen shot for comparison.

Before Firefox crashed and I had to reinstalled it everything worked correctly. I use Eset. I did add an exception for Amazon and had to turn on background images when you right click on the text page. But that option is not available on weatherunderground. I've enclosed the certificate images similar to your example that I get from the aforementioned site. What's confusing to me is Firefox is pretty good at not removing your settings and everything I can find and set/unset does not work on this web site.

The same results are when I try ti do Firefox Help. Text only. Here is the certificate info:

please try the steps mentioned for eset at https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER#w_eset

Thanks. That did the trick. Learn something new every day.