There’s an addon (use the search term favicon) that will provide a favicon for a link without one. If opening the folder works, you can give it a new icon. Other than that, Iike using Google Bookmarks, in which you can file one site under many different tags. Not the same, I know, but still useful. Also look for an addon called Smart Toolbar which removes the text but allows it to pop up when you hover, so you have an all-icon toolbar, but you can still have labels.