Remove all Firefox bookmarks at once, using the Library dialog. Press Ctrl+Shift+b on your keyboard navigate to Bookmarks > All Bookmarks
From the left pane, click Bookmarks Menu or the location of the Bookmark you would like to delete.
In this example I’m deleting all Unsorted Bookmarks.
Once selected on the right panel, select any of the bookmarks and press Ctrl + A on the keyboard to select all, then right-click > Delete.