When you get this for pretty much all secure sites, the problem usually 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, ESET, Kaspersky.

(3) Malware on your system intercepting secure connections.

So... which is it?

If you have any of those specific security products, that would be the first thing to check. We might be able to assist with specific next steps based on what you have if you tell us.

Alternately, 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.

Since so many people use the free version of Avast, I'll just mention this now.

Avast Web Shield intercepts your browsing and filters it. To filter secure sites, it presents "fake" certificates to Firefox. Avast should have set up Firefox to trust its fake certificates but that seems to fail on a regular basis.

To test this theory, try turning off scanning of encrypted sites. I saw these steps in another post:

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, but you prefer to filter encrypted communications, you may need to import the Avast! signing certficate into Firefox's Certificate Manager, Authorities tab.

If you can't inspect the certificate via "I Understand the Risks" then try this:

Open the "Add Security Exception" window by pasting this chrome URL in the Firefox location/address bar and check the certificate:

• chrome://pippki/content/exceptionDialog.xul

In the location field of this window type or paste the URL of the website.

• retrieve the certificate via the "Get certificate" button
• click the "View..." button to inspect the certificate in the Certificate Viewer

You can inspect details like the issuer and the certificate chain in the Details tab of the Certificate Viewer. Check who is the issuer of the certificate. If necessary then you can attach a screenshot that shows the certificate viewer.