You don't have Avast, but there are other possibilities. I'll paste my whole spiel:

When you get "unknown issuer" for pretty much all secure sites, the problem usually is one of the following:

(1) 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.

(2) 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)

(3) 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.

I do not have any of those security softwares. I ran the test page and got this as the issued by: VertoAnalyticsCA

I fixed it by deleting the verto smartpanel app from my computer. Thanks for your help!

Do you now or have you ever had this on your system: Smart Panel or Smart App from https://vertosmart.com/

Unless you want them watching all your browsing, I suggest removing that software.

Then, to clean up your connection:

(1) Proxy Setting

If you check Firefox's connection settings, do you notice anything unusual? Under:

"3-bar" menu button (or Tools menu) > Options > Advanced

Click the Network mini-tab, then the Settings button. The default of "Use system proxy setting" piggybacks on your Windows/IE "LAN" settings, but you also could try "No proxy" to see whether that makes any difference.

(2) Clean Windows Network Settings (if needed)

If the interception software is not running any more, then Firefox should connect directly. If it still can't connect, you might need to shut down/restart Windows, or perhaps there's another piece embedded in the Windows networking settings, for example, a Layered Service Provider or LSP.

There is a procedure to "reset" the Windows networking components, discussed in this thread: After upgrade to window 10, cannot access internet from firefox or chrome. I don't know whether that would help or cause new problems.

Hey, you beat me to it! Congratulations.