As you probably know, Safari allows you to rearrange the entries on your bookmarks bar via a simple drag-and-drop. You can move both individual bookmarks as well as bookmark folders using this method, and it works great. But what if you’d like to move a bookmark (or bookmark folder) on your bookmarks bar into a folder on your bookmarks bar? You could use the Bookmarks -> Show All Bookmarks menu, of course, but there’s a simpler solution.
To move a bookmark or bookmark folder into another folder on the bookmarks bar, click-and-drag on the entry you’d like to move—but drag vertically, not horizontally. When you move your mouse down, you’ll find that you’ve pulled the selected item off the bookmarks bar. If you release the mouse button now, the dragged item will disappear in a puff of smoke—so don’t release the mouse button.
Instead, drag the selected item up to the bookmarks bar again, and position it over the destination folder. When you do, the folder will highlight—release the mouse now, and the dragged item will be filed within the highlighted folder. I find this much simpler than using the Show All Bookmarks menu item.