I am able to visit www.national-lottery.co.uk with Firefox without seeing any errors. Problem may well be local and on your computer.

• Check the computer's date and time are correct.
• Are you using
  • A private wifi site or some proxy server
  • Any security software that could interfere with certificates. Please list all security related software that is in use. (Avast may cause problems)

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.

If that didn't help then remove or rename secmod.db (secmod.db.old) as well.

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

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

Thanks to john99 and cor-el for your replys. john: time and date are ok, private secure wi-fi and avg security and norton expired. No changes to these and until now no problems accessing the lottery website, cor-el: will take a look at your suggestion and report back.

As a matter of interest I followed links (blog and SSL version control) on the error box which when installed, allowed access but problem re-occured and a second install didn't work, Best Regards, Rob.