Don't think so RPGFX, the icons that appear on the bookmarks toolbar are controlled by the web site itself.  If there is a favicon.ico file in the root directory of the site and it is specified to be used by the site coding, that is what appears as the icon.  If there is no icon designated at the site, Firefox will just use a generic icon instead.

I recommend getting rid of the favicons completely. I don't like the way they clutter the toolbar. Then, if you add flexible space on both ends of the toolbar, it will center the bookmarks and it will look even cleaner.

Install this add-on if you want to try it out. LINK

That's what I normally do, G3mpi3... but I figured if I could use my own created favicons, then I could theme them.  Oh well, thanks guys!