I've now tried several times, Firefox (here V3.6.12) closes with each click (right or left) the window.
Yes, Firefox works that way. And we can't do anything because of that.
If you set $closeByClick=0;
this will let the script to disable the closing the window on click, in any browser. This is the common solution for all browsers. If some users want to download pictures, let them use another browser besides of Firefox. I think for those who really want to save pictures, it wouldn't be a problem... I also think, this is a minor part of users not to specially care :-) The most important is to let users act more comfortably, and I think, closing the pop-up on click is a more comfortable and quicker way.