If you've added both bookmarks and folders of bookmarks to your Bookmarks menu in Safari, and have Tabbed Browsing enabled in Safari preferences, when you choose Open in Tabs from the Bookmarks menu, the outcome will depend on which version of Safari you have.
In Safari 1.3 and later (version 1.3 is included with Mac OS X 10.3.9; 2.0 or later is included with Mac OS X 10.4 "Tiger"), the "Open in Tabs" behavior is different than in previous versions of Safari. Prior to Safari 1.3, choosing this command from the Bookmarks menu would open only the individual bookmarks that appeared in the menu, and not the ones contained in your folders or subfolders in the menu.
In Safari 1.3 and later, this command opens both the individual bookmarks and those contained in folders. Therefore, choosing this command directly from the Bookmarks menu isn't a good idea if you have a lot of folders with lots of bookmarks in your Bookmarks menu. Here are a couple of ways to adjust to this new behavior:
Note: The "Open in Tabs" command will not appear in the Bookmarks menu if you haven't selected the Enable Tabbed Browsing checkbox in Safari preferences.