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Anon, this is because of firefox changing their how they handle referrers and what information a website is given. The site generates a temporary image link to prevent permanent links (as others used Sankaku Complex for image hosting for their websites), and your referrer information contains something that identifies you temporarily so you can actually use that image link.
Private browsing strips this information so the website thinks you're just some random person who got this hyperlink and they give you a tiny error image as a result.
You can revert this by going into your firefox configuration and edit the value for "network.http.referer.defaultPolicy.pbmode" to 3.
