Hello,
I'm having the exact same issue on my computer running Win7 x64 and firefox 18.
I tried with juicebox lite v1.2 on my computer.
I'd like to buy it but have this is quite a big problem.
It's doing it with my own galleries and on this demo too (when clicking on thumbnails : http://www.juicebox.net/demos/lite/full/

Thanks for your reply