I too am having an issue with Macs.
The page I created has worked since April when the site was created, now all of a sudden it does not load. I can view other SimpleViewer pages fine, just not my own. It tries to redirect me to a URL graphics/upgrade.html instead of graphics/index.html.
The page works in Firefox and IE on PCs, but not on Safari or Firefox on Macs.
I started looking into SimpleViewer after seeing it on a couple of sites. Since last week (July 22 '06), when I return to these sites the gallery will not load. The page redirects to upgrade.html. I can confirm that this is happening after the last upgrade to Firefox on the Mac and that the sites do play properly in IE6 and Firefox 1.0.3 on the PC. I'm wondering if this type of problem might affect sites again in the future.