Your Question Details > More System Details indicates a number of Kaspersky add-ons, so...

Did you recently use Firefox's Refresh feature? The tell-tale sign is a new "Old Firefox Data" folder on the desktop (and/or a new semi-randomly-named Firefox settings folder inside).

After a Refresh, Firefox's certificate store is cleared and Firefox no longer trusts the fake certificates presented by Kaspersky in connection with the filtering of SSL connections. You can reestablish trust by importing the Kaspersky signing certificate. Or if you refreshed recently, by copying a file from the old settings folder to your current one.

Before providing many paragraphs of further detail, can you confirm this is a Kaspersky issue? You can examine the certificate to which Firefox is objecting to see whether the issuer information points to the culprit. Take my test page for example:

https://jeffersonscher.com/res/jstest.php

You should see an "I understand the risk" heading in the page. If you expand that section, you should find an Add Exception button. 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, then View. If View is not enabled, try the Get Certificate button first. Then in the Certificate Viewer, look at the "Issued by" section. What do you find there, and/or under Certificate Hierarchy? I have attached a screen shot for comparison.

I don't remember if I used the refresh feature or not. I don't even know what that is. How do I import the Kasperky signing certificates? I didn't find an "I understand the risk" heading on the page you provided but I did follow your instructions at the very bottom and found that the "issued by section" said: generated by avast! antivirus for SSL/TLS scanning.

That's confusing. Do you have Avast installed as well as Kaspersky? It's generally most stable to only use one "real time" antivirus scanner at a time.

More info for Avast:

The Avast Web Shield intercepts your browsing to scan for threats. Intercepting secure connections requires presenting "fake" certificates to Firefox, and unless Avast's installer set up Firefox to trust those certificates, you get errors on all secure sites.

You could try switching off scanning HTTPS connections to see whether this is the cause in your case. These steps are for the Windows version, but probably similar on Mac:

1. Open the Avast dashboard on the affected system.
2. Select Settings from the left sidebar menu.
3. Switch to Active Protection.
4. Click on Customize next to Web Shield.
5. Uncheck the "Enable HTTPS Scanning" option and click ok

If that resolves the issue, you can still use the feature if you can get Firefox to trust Avast's signing certificate. You could try reinstalling Avast to trigger that process.

It worked! Thank you so much!