Check your system date and time, although maybe that would affect other browsers. Obviously all sites are not using invalid certificates, so it must be something else going on. If the issue is due to security related software maybe some component of that needs to be uninstalled rather than just turned off.

Also see

• What do the security warning codes mean?
• How to troubleshoot security error codes on secure websites_the-error-occurs-on-multiple-secure-sites

I have made sure all the time and date is correct. I have my suspicions that it isn't to do with the certificates on the site, more to do with something stopping the traffic.

I have run an AV scan to check for anything malicious. Nothing has come back, i then uninstalled the AV, still no luck with firefox. I have had a look at some suggestions with importing the SSL certs from another browser but that hasn't worked either - ready to pull my hair out!!!

There is security software like Avast and Kaspersky and BitDefender and ESET that intercepts secure connections and sends their own certificate or that incorporates special web shielding features that can block content.

If you can't inspect the certificate via Advanced (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 with the https:// protocol prefix (https://xxx.xxx).

• 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 please attach a screenshot that shows the Certificate Viewer with the issuer.