When you create a folder for bookmarks in the Bookmarks window (Command-Option-B to open it), and put that folder on the bookmarks bar (by dragging it into the Bookmarks Bar collection), you get drop-down menu on the bookmarks bar listing those bookmarks. Change it to an “auto-click” folder, and instead you get a button that, when clicked, opens every bookmark in the folder, each in its own tab in the current window.
To make an auto-click folder: working in the Bookmarks window, select the folder in the bookmark pane and click the checkbox in the Auto-click column.
You can differentiate regular buttons from auto-click folder buttons in the bookmarks bar by looking for the little box to the right of the button name that identifies an auto-click button.
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