If you close the sidebar then this bar should stay closed when you restart Firefox. *View > Sidebar > Bookmarks You can check for problems caused by a corrupted localstore.rdf file. *http://kb.mozillazine.org/Corrupt_localstore.rdf Note that current Firefox releases have a Sidebars toolbar button in the Customize palette. *https://support.mozilla.org/kb/customize-firefox-controls-buttons-and-toolbars ---- Opening in the sidebar 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 sidebar (Ctrl+B; Command+B on Mac). In the Bookmarks Manager (Bookmarks > Show All Bookmarks) you can click the More button in the Details pane at the bottom right. Make sure that "Load this bookmark in the sidebar" is not selected.