Clicking https://i.imgur.com/nZHfzdV.jpg yields "Sign in required - This page may contain erotic or adult imagery. You'll need to sign in if you still want to view it.".

Opening the same URL by copying and pasting it to the address bar shows the image without needing to sign in.

Imgur does something goofy with the referer that you've clicked a link from. I'm by no means a competent web programmer or understand how linking/referrers/etc. work but at one point I had a little doodad in GreaseMonkey/Tampermonkey that pulled imgur images in and plopped them on your Facebook page.

What I found was that if the page referrer was Facebook, you got nothing, the imgur site attempted to redirect you back through to their galleries, but if you used the link directly you'd get the image.

I never figured out exactly what's happening, I used a code snippet that stripped out the referrer info and everything started working.

I think you can do it without extensions in firefox since they let you configure your own referrer-policy: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Re... (search for "about:config")

But even in my own APIs I check for referrers to make sure they are actually coming from my site so I assume this will break some pages.

See also - Smart Referer[1] firefox extension. Only blocks cross-domain referer leak, and has a built-in whitelist to prevent breakage.

^1: https://addons.mozilla.org/en-US/firefox/addon/smart-referer...

Maybe they're also using the Accept: header, which would reveal whether the browser is loading a page or an embedded image?

Not if you right-click and open in a new tab. Must be a referrer thing.