my image doesn’t show anything… neither http nor https, I didn’t capture the address bar ;D
that’s the link I used: https://blog.avast.com/2011/09/16/windows-8-conference

… okay to be clear I used an https link, and there’s nothing weird that you don’t get any alert in Firefox (I don’t either ;D ) as this is a Chrome feature (to send such popups). On the other side Firefox should also show a warning in the address bar when clicking on the site icon in the address bar, it should also tell that there are http elements (non-ssl). Strangely it doesn’t, even with NS and AB+ disabled.