Neither of those options are ticked....

I managed to do what I wanted at least by adding this:

hr.catBoxDivider1, hr.catBoxDivider2, hr#catBoxDivider {
border-color: #e7e7e7;
}

............. to my stylesheet_categories_dressing.css

You're right though, Firebug does seem to be computing it's own associated styles.. even though neither option is selected in Firebug Options Menu.