Why am I suddenly getting a “Blocked loading mixed active content” issue in Firefox?

Question

This morning, upon upgrading my Firefox browser to the latest version (from 22 to 23), some of the key aspects of my back office (website) stopped working.

Looking a

Answer 1

In absence of a white-list feature you have to make the "all" or "nothing" Choice. You can disable mixed content blocking completely.

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The Nothing Choice

You will need to permanently disable mixed content blocking for the current active profile.

In the "Awesome Bar," type "about:config". If this is your first time you will get the "This might void your warranty!" message.

Yes you will be careful. Yes you promise!

Find security.mixed_content.block_active_content. Set its value to false.

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The All Choice

iDevelApp's answer is awesome.