How to move bookmarks *with icons* from Firefox to Safari
Safari doesn't show icons for bookmarks in the toolbar itself, only in the bookmark menus.

The icons aren't stored with the bookmarks, Safari has a separate cache for all the favicons of sites that have been visited (I believe this is the case for Firefox and Chrome as well, but I'm not 100% sure). If you look in the bookmarks viewer or menu, open a site that doesn't have an icon, then check back in the bookmarks viewer, you'll see that Safari has added the icon to that bookmark entry (assuming the site has a favicon set in the first place). So while you may not see icons at first for bookmarks you import, they'll fill in eventually as you visit the sites.