Opening in the side bar is the default for bookmarks that are created via a link or button on a website. You can check the Properties of a bookmark via the right-click context menu in the side bar (Ctrl+B). In the Bookmarks Manager (Bookmarks > Organize Bookmarks) you can click the More button in the Details pane at the bottom right. Make sure that '''"Load this bookmark in the side bar"''' is not selected.