Security certificate is controlled/validated by firefox when you visit HTTPS websites, not HTTP, so it shouldn't say that. Also, the provider doesn't appear to use secure transfer protocol right now, since requests for http secure connections fall back to http.
There's a troubleshooting guide mentioning this problem here:
Your issue could be the same as in:
If you see this heading without accessing a secure site, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.