Insecure content
Off Topic
Your looking for the right thing.
Here's what I found:
This explains the issue:
https://support.mozilla.org/en-US/kb/how-does-content-isnt-secure-affect-my-safety?as=u&utm_source=inproduct
(which you already know.)
This is someone else in the same situation:
http://stackoverflow.com/questions/4728507/finding-all-insecure-content-on-a-secure-page
This was the suggested next step:
"Why No Padlock?"
https://www.whynopadlock.com/check.php
When your site was put in, it returned:
Quote: Valid Certificate found.
Problem verifying certificate.
Problem verifying certificate.
It started to get very specific and technical when searching for
Quote: why not padlock "Problem verifying certificate."
does "Problem verifying certificate" mean anything to you? To me it seams like an issue with the servers SSL Cert. Don't have many more clues than that though.