If there is a padlock with a red strike through at the left end of the location/address bar then that means that mixed active content is blocked.

• https://support.mozilla.org/kb/how-does-content-isnt-secure-affect-my-safety
• https://developer.mozilla.org/Security/MixedContent

If there is mixed passive content (e.g. images) then Firefox shows a padlock with an exclamation mark attached on the location/address bar.

• https://support.mozilla.org/kb/control-center-site-privacy-and-security-firefox

See also the Web Console (Firefox menu button or Tools > Web Developer) for messages.

• https://developer.mozilla.org/Tools/Web_Console
• https://developer.mozilla.org/Tools/Network_Monitor

I found the right one on a link on one of those links:

Doesn't fix it though, it just means I can't buy anything on Ebay, Amazon or Forbidden Planet.

https://support.mozilla.org/en-US/kb/how-do-i-tell-if-my-connection-is-secure#w_gray-padlock-with-yellow-warning-triangle

Gray padlock with yellow warning triangle "A gray padlock with a yellow warning triangle indicates that the connection between Firefox and the website is only partially encrypted and doesn't prevent eavesdropping.

For information about what "partially encrypted" means, see Mixed content blocking in Firefox. This is a problem the site developer needs to resolve.

Note: Do not send any sort of sensitive information (bank information, credit card data, Social Security Numbers, etc.) to sites where the Site Identity button has a yellow warning triangle icon. "