^ Ahh, yes. It is a browser misconfiguration. But it's not your fault!
First some background (you may already know this stuff, so bear with me here).
It is standard practice for web resource requests to provide a URI and a with it a referer. Your browser, when it asks for something (such as an image) says to the website "this is the name and address of the thing I would like you to send me" (the URI) and it also says "and here is the name of the place which sent me" (the referer).
So when you load this page and your browser gets to the first image I posted, it sees that the image lives on my website (not AusPhoto) and it sends a message to my web server (a) asking for it by name, and (b) saying which website is asking for it (auspgotography.net.au in this case).
My webserver (a) looks to see if the image exists, and (b) looks to see if the website asking for it has permission to access it. If the file exists and your request is a legitimate one, it sends the image and your browser displays it on this page.
This is worldwide industry standard practice and has been for many years. But why does my web server check for permissions? Couldn't it just hand the image out to anyone who asks?
No. It is an essential security measure.
Suppose someone wants to steal a picture and display it on a popular high-traffic website? He could download or screen shot the image, upload it to his own server (if he has one) and display it from there. That's illegal but easy enough to do, and there is nothing I can do to stop it short of taking legal action. But in reality, I'd probably never even know it had happened, and in truth I wouldn't really care too much about it. Stuff happens. Meanwhile, every time someone opens the page where he displayed the image, their browser asks his webserver to send the picture, which it does.
But what if he does it the easy way? Simply types something like <img>http://mywebsite.au/mypicture.jpg</img>? Now it is MY website that serving up the picture to his readers. This is called HOTLINKING or BANDWIDTH THEFT and it can be very serious. If he has put my picture on a high-traffic site it can blow my monthly bandwidth allowance out of the water in minutes. At that point, my hosting company automatically shuts my entire site down, and also any other services on the same account (such as my email and any other sites I have). They have to do that to protect their other sites from the bandwidth hog.
This is why any properly configured web server checks the "referer" (yes, that weird spelling is correct - someone decades ago got it wrong the first time and now we can't change it without breaking stuff) to see whether the resource request is coming from a trusted site. What is a trusted site in this context? Obviously itself (for example the abc.net.au server will happily hand out images if the requesting site is also abc.net.au), but also any other site the administrator put on his or her "safe list".
So when AusPhotography asks for a picture from my server, your web browser sends a message saying "send me Tannin's picture such-and-such, I'm from ausphotography.net.au", and my server sees that AusPhotography is on the list so it sends the picture.
This is all standard practice. I've used my own example but it is similar for pretty much every web administrator of every site. (But note that some of the really huge Internet companies - the likes of Facebook and Google - use their own proprietary methods instead of referer checking. Their methods are undisclosed.)
Could someone fake a referer and trick my server into sending the picture anyway? Sure, it's not that hard to do. But who would bother? And even if they did, that is only one user - to do any real harm (such as shut a server down) a hotlink has to involve thousands of users. So checking the referer is good enough for almost all practical purposes, and all web browsers since 1996 (28 years ago!) respect it.
Now we get to the problem. The idiots in charge of Firefox decided to break the rules. Pale Moon, Sea Monkey and Waterfox (all Firefox forks) now have the same problem. Every other browser (Opera, Vivaldi, Chrome, Brave, Safari, even Microsoft Edge, plus whatever other ones I haven't thought of) continues to work just fine. Simply, it is a Firefox problem. It only happens to the (roughly) 5% of all Internet users who use Firefox or one of its relatives. The (roughly) 95% using non-Firefox browsers are fine.
Now you could fix it by getting rid of Firefox, but it's a good browser. I like it and use it myself (though not for Aus Photography) and also use Pale Moon every day, and sometimes Sea Monkey and Waterfox for particular sites.
Or you could fix it by going into about:config and changing some settings, but that would only fix it for you. Every other Firefox user would still have the problem. We can't fix their browsers!
I reckon there might be a workaround. I'm going to try using a secure connection link to an image. That is totally unnecessary as there is nothing remotely private, confidential, or in any way security-related anywhere on my site (other than my administrative & management interface, which we are not interested in here) and rather annoying to have to do just because Firefox (5% of the Internet) thinks it can break a long-established rule honoured by the other 95%, but that's the way it is. The Firefox developers have form so far as deciding to do their own thing and to hell with the rest of the world - that's pretty much why that once-time world-leading and dominant browser is down to single-digit market share these days.
If you see a picture below (another landscape with the 100-400 II, as it happens) then my workaround for the error in Firefox works. Wish me luck!
https://tannin.net.au/upload/13/130224_120025_1d4x.jpg