Do you use Avast anti-virus? Is your Windows 10 account a child account? Both of these things will cause this issue.

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.

babs7879 said

This Connection is Untrusted You have asked Firefox to connect securely to www.google.co.uk, but we can't confirm that your connection is secure. Normally, when you try to connect securely, sites will present trusted identification to prove that you are going to the right place. However, this site's identity can't be verified. What Should I Do? If you usually connect to this site without problems, this error could mean that someone is trying to impersonate the site, and you shouldn't continue. This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox only connect to it securely. As a result, it is not possible to add an exception for this certificate. I have tried all the steps listed on the site but i still get this message. I have recently updated to windows 10. would this cause this problem?

Its a child account on windows 10 Can I get round this?

If it is a child account, there are two options:

Follow the steps on https://support.microsoft.com/en-us/kb/2965142 (they are for windows 8.1 but apply to windows 10).

Or remove the child restriction from the account.

while in firefox, you could also try to click on this link in order to install the missing certificate - then a dialog about adding a cert should display. please check the box to Trust this CA to identify web sites & confirm the change...