That's because Firefox only warns is the page is completely insecure, Chrome flags a whole page as insecure if there is at least one insecure element in the page as it leave the secure elements vulnerable to known exploits.
Not sure why all the links redirect back to plain HTTP either. As I said originally, bit of dodgy config.
Certainly could be improved by the site admins, the SSL implementation isn't great for the secure side either.
Report here: https://www.ssllabs.com/ssltest/anal...=airgunbbs.com