You can check the date and time and time zone in the clock on your computer: (double) click the clock icon on the Windows Taskbar.

• https://support.mozilla.org/kb/Security+Connection+Failed

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

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.

I checked the time and date and they were correct, I do use Kaspersky Total Security as anti virus and firewall protection, I did inspect the Certificate Viewer before and that is how I identified the issuer as "thawte DV SSL CA - GA". I'm unable to do a screen shot as I'm using a different computer and don't want to risk infecting it. I did use Google Chrome earlier to post this question and it was working fine, but when I tried to go into it just now it was blocking me from accessing secure urls.

Try to rename the cert8.db file (cert8.db.old) and delete the cert_override.txt file in the Firefox profile folder to remove intermediate certificates and exceptions that Firefox has stored.

If that has helped to solve the problem then you can remove the renamed cert8.db.old file. Otherwise you can rename (or copy) the cert8.db.old file to cert8.db to restore the previously stored intermediate certificates. Firefox will automatically store intermediate certificates when you visit websites that send such a certificate.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that helps.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Thank you cor-el. I used your first suggestion of renaming the cert file and deleting the cert override file, it seems to have solved the problem on the websites that I was getting the unsafe messages. One query I have is do I need to copy the old file or can I just leave it as it is? Also I noticed that the cert override file hasn't reappeared, is this usual.

If the new cert8.db that Firefox created works then you can delete the old file that contained certificates that were possibly corrupted or outdated.