getElement('class') doesn't work in IE, while getElement('className') doesn't work in FF. Instead, use object notation, i.e. element.className, which works in everything.
In your first post, loadImageInPhotobox() never defined the var 'thumb' but you seem to have sorted that as, in your second post, loadImageInPhotobox doesn't call that var.
If you still have no joy, post your HTML too.