I'm helping debug this issue for africansafari, and it looks to me like a conflict between Prototype and jQuery. The various lightbox scripts are having a good old whinge about "$ is not a function" in the Firefox error console.
The site is using forms on most pages, which as I understand introduced the prototype code into the mix. We are already using jQuery.noConflict(); to get Prototype playing nice with jQuery (which we use for another script), however *most* of the lightbox scripts in the image_gallery plugin are using the $() notation rather than jQuery() notation so noConflict won't help here.
The ShadowBox option uses jQuery() though. So for everyone else, we managed to solve (or at least workaround) the issue by switching the gallery to Shadowbox, and making sure that the following code is inserted after jQuery has loaded:
Hope that helps someone. I'm open to any better suggestions by the way - I understand SS are trying to remove Prototype in a future release, would be nice to slim down the site by not loading 2 competing JS libraries.