Google.co.uk will not open due to certificate not being recognised, help

Question

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.

Answer 1

Check the date and time and time zone in the clock on your computer: (double) click the clock icon on the Windows Taskbar.
Check out why the site is untrusted and click "Technical Details" to expand this section.
If the certificate is not trusted because no issuer chain was provided (sec_error_unknown_issuer) then see if you can install this intermediate certificate from another source.
You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.
*Click the link at the bottom of the error page: "I Understand the Risks"
Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".
*Click the "View..." button and inspect the certificate and check who is the <b>issuer of the certificate</b>.
You can see more Details like intermediate certificates that are used in the Details pane.
If <b>"I Understand the Risks"</b> is missing then this page may be opened in an (i)frame and in that case try the right-click context menu and use "This Frame: Open Frame in New Tab".
*Note that some firewalls monitor (secure) connections and that programs like Sendori or FiddlerRoot can intercept connections and send their own certificate instead of the website's certificate.
*Note that it is not recommended to add a permanent exception in cases like this, so only use it to inspect the certificate.