1. First install a firefox add-on called "Stylish 1.0.3" from
https://addons.mozilla.org/en-US/firefox/addon/2108 by click on "add to firefox" icon, and restart firefox.
2. Then go to
ToolbarButton icons- On Bookmarks Toolbar ANY SIZE | userstyles.org (ToolbarButton icons- On Bookmarks Toolbar ANY SIZE) and scroll down to see the install options, click on "load into stylish". Then restart firefox, you should see the icons become smaller then before. You don't have to do any size change since the default for this style is smaller icon size
Edit: do you mean you want the icon without the text!?