I guess I should have said "something wrong with the way the viewers are interacting with your particular server".
But let's move on. Felix sent me the url for your site so I could take a look on my Mac. I can see the problem in Firefox on the Mac when I browse to your galleries. Safari and Opera seem ok.
So I took a closer look with Firebug. Your server is returning a 403 Forbidden response when the SimpleViewer Flash file sends a request for the images. When I browse directly to the images with Firefox then I see the error page right there in my browser. I'm typing the image url directly into the browser so svManager and SimpleViewer are out of the loop. Yeah, I know, svManager put the images there and that may be relevant – I'm not forgetting that.
Strangely, I can browse to the images in Safari. This is the first time I have ever seen a server refusing to serve images to one browser and not to another.
So we seem to have some rather unusual server behaviour – where do we go from here?
Firstly, please check that you are seeing the same as I am when you browse directly to the images, i.e. an error page in Firefox and an image in Safari. I'll assume for the moment that you are.
Secondly, I would make a note of the permissions on all the relevant folders and files before anybody changes anything.
Thirdly I would contact your server support or helpdesk and explain that you can browse to certain images on your server in Windows and Mac Safari but not in Mac Firefox. I wouldn't bring svManager or SimpleViewer into it – I'd concentrate on the core problem of the error page. It might help them if you quote the permissions you just noted down.
Next I'd wait a bit before you change anything to see if you get any help from the server support. If not then you could experiment with changing permissions on the image folders and the images themselves to see if you can eliminate the error message.
Is it a Unix server or Microsoft? I could maybe give you a little more help on permissions but like I say, I've never seen this before so I would be feeling my way and this post is long enough for the time being.
SimpleViewer Support Team.