Thanks for your quick reply!!! And I've found the error. It was because of the twig debugging system that was active. Once it was disabled everything worked as a charm.

The juicebox gallery works perfect on chrome, Safari,... but on Firefox (mac) the photo is too big and there are no thumbnails on the page, just a black banner with the number of images and a link 'view gallery'. When I click the link I only see all the thumbnails. When I click them I only see a white page. When I hover over it I can see the arrows and the three links in the upper right corner....
I added a printscreen.