I finally got it with Web Developer Tool in Firefox. There were 4 files in includes/modules/pages/product_info/ which couldn't ne found returning 404 header:
builder.js
dragondrop.js
control.js
slider.js
IE didn't have problem with not finding them and loaded product pages quickly but Firefox hesitated, hanging a while longer that is acceptable. The file calling them is jscript_icmain.js which came with HoverBox 2.1
Here is en excerpt from this file. It didn't use "builder.js" but just "builder" and this is why I couldn't find builder.js when searching for it via Developers Kit in Admin.
...(includes?includes[1]:'builder,effects,dragdrop,controls,slider').split(',').each(function(include){Scriptaculous.require(path+include+'.js')});});}};...
I removed 4 items higlighted in red (the effects.js script was the only one actually present in the directory) Product pages in Firefox started loading quickly. The HoverBox doesn't need these files and the script above shouldn't be calling them. I am posting this for all those who use HoverBox 2.1 and experience slow loading of product pages.
Thanks to all who were trying to help!:clap: