Hmm, I don't see any warning for that site. Are you seeing the warning page for that feature (pictured below), or something a bit different?

Thanks for your reply. Afraid I didn't notice it before and have just posted another query on this topic as the problem is recurring with other safe sites, including a French government website that I'm sure is perfectly safe and shows as https with a padlock by it when I proceed against Firefox's warning message.

Yes, that's the message I get.