[Mac OS X Hints] The final version of Safari introduces Auto-Tabs. An Auto-Tab is a folder of bookmarks that behaves as one bookmark — when clicked, it will open all the URLs in the folder in separate tabs. To enable Auto-Tabs for a folder, open the Bookmark view and then find the folder you’d like to modify, and then just check the Auto-Tabs box. Once checked, the little triangle in the folder’s name will become a little square, and now your folder acts like a single URL bookmark. Quite handy.
To bring back the dropdown menu, just press Option key before you click on the auto-tabbed folder; you’ll get the standard dropdown list of all the URLs in the folder.