I've done a lot of work on this, and here is the latest summary:
I've disabled, one by one, each of my Firefox (12.0) add-ons. Then I'd test this problem, then enable the disabled add-on, disable the next one, and repeat. You can see a list of the 11 enabled add-ons I started this test with here: http://www.billanddot.com/firefox-add-ons.jpg . But, as I'll attempt to explain below, I don't think this is an add-on problem.
This test didn't help solve the problem. I really think this is a timing problem within Firefox, and so because some of these add-ons may be more "intensive" than others, I think this may effect how often I have to Refresh Firefox in order to see the partial-image-load problem. And this is why, I think, that I can't duplicate this problem in Firefox's Safe Mode: Firefox then has too little extra work to do, and so responds too quickly for this timing issue to show up.
In case you haven't followed me, I first noticed this problem doing a Gallery -> Preview. However, other times, this works fine, i.e., that first image loads fully. (I've never had a problem with partial loading looking at images 2 - N.) This led me to experiment with using Firefox's Refresh on the local file that Juicebox sent as the URL to Firefox. Pressing Refresh repeatedly - that is, right after I can see that that first image has just finished loading - reveals that the problem sometimes manifests itself, and sometimes doesn't. And that leads me to believe that this is mostly a Firefox problem, because pressing Refresh has nothing to do with Juicebox at that point, right? I suppose it could conceivably have something to do with the actual code that Juicebox generates, and how Firefox interprets that code.
Now, to show you exactly what I'm talking about, I made a video of this situation, and uploaded it to YouTube. It's 35 seconds long, is in real time (no video editing), and is here: http://www.youtube.com/watch?v=NNKp0UHMYs8 . What you will see here, replete with mouse movements and clicks, is:
- I invoke Gallery -> Preview, and Firefox (already running) doesn't completely load that first image;
- I then switch my attention from Juicebox to Firefox, and repeatedly press the Refresh button;
- Most of the time the local page/first image is refreshed fine, and the image loads fully, but a couple times the image does not load fully;
- On the last such partial-load, I hover the mouse (but do not click anything) over the bottom thumbnail area, which hovering magically results in the completion of that partial load (i.e., the image is now fully loaded).
Fin.
I'm not about to disable all my add-ons when I want to preview a gallery; if the preview doesn't load that first image fully, all I have to do is move the mouse down to the thumbnails. So it's not a really big deal. Nevertheless, I'm convinced that there is a bug, somewhere, in Firefox itself, or in the way Firefox handles the code generated by Juicebox, or some combination of this. So I'm reporting it, but I'm done working to isolate it, because I really can't and I haven't got the time.
Hope this helps,
Bill