Firefox: "Some parts of this page are not secure, such as images." What counts as insecure?

Anything that is delivered over an insecure channel.

What this generally means is that the developer of the web page is combining HTTP-based URLs with HTTPS-based URLs in the same page. The URLs could be for images as well as JavaScript, CSS, or anything else that can be referenced from a web page. As a user, there's not much you can do about this -- it's a warning that there is a possibility that your data could be delivered to other servers in an open, unencrypted manner over the Internet. This is a Bad Thing, but you can't do much except avoid that site, or contact the support or webmaster for the site.

If you're the developer, most of the time you can use a scheme-relative URLs when referencing images or javascript, etc.

i.e. Instead of this:

<img src="http://example.com/dot.png">

use this:

<img src="//example.com/dot.png">

YMMV.

See also: https://url.spec.whatwg.org/

In firefox you can see in Inspect Element=>Network Tab=>Domain Columns. And also please check in Console tab too.

I hope it will solve your problem.

"Insecure" simply means "not loaded via HTTPS".

This is insecure:

<img class="media-object" src="http://placehold.it/50x50">

This is secure:

<img class="media-object" src="https://placehold.it/50x50">