Dear Admin,
i've tried the solution with disabling the anim as described under

http://floatboxjs.com/forum/topic.php?id=261

i disabled it by modifying floatbox.css with replacing the entry with the link to the anim-gif to

#fbx.fbx .fbLoader{display:none}

That helped so far, that the image isn't displayed anymore, but the empty loader-box still comes up for a short when load time is too long before the final floatbox opens.

How to disable the empty loader box completely and show only the final box when it's done?

I don't have a supported mechanism for not showing the loader gif while the box is loading. Modifying the css is really a hack in Floatbox's world and what works for one version may well not work for another.

Probably the easiest hack for this would be to replace the loader.gif file(s) in the graphics folder with blank.gif. Then loader.gif will still show; you just won't be able to see it.

I'll put a showStartGif option in the version 6.0 release because it seems there's at least two people who want this.