Even if the problem is only with a certain browser, we would still very much like to track the problem down.
If you have the time, I would be grateful if you could please confirm whether or not you see the problem in my test gallery (which is a gallery straight out of JuiceboxBuilder-Pro but using the same configuration options that your own test gallery uses).
The fact that you do not see the problem in any of our demo galleries suggests that the problem is either with a certain combination of configuration options or with custom code on your own web page. If you do see the problem in my test gallery, then the configuration options used in the gallery may be the cause (and I can then log a bug with the developers if necessary).
Thank you.