What they did in Firefox 4 is overhaul the entire interface including the bookmarks toolbar. They now have a bookmarks toolbar that shows up with the bookmarks you put on it plus the bookmarks menu button to get to all of your other bookmarks easily. When you hide this toolbar you'll get a bookmarks button behind the search box. When you show the bookmarks toolbar, the toolbar comes up and the button behind the search box will disappear.
Customising the bookmarks toolbar won't work since it will remove everything on that toolbar. What you need to do is first hide the bookmarks toolbar. You'll get the extra button behind the search box. You then need to customise the toolbar and remove this button. When you switch the bookmarks toolbar back on, all your bookmarks are still there and the bookmarks menu button has disappeared.