Your screenshot shows the menu bar and not the sidebar. You should be able to close the actual sidebar that can show bookmarks and history via a close X that appears at its top right corner. *https://support.mozilla.org/en-US/kb/use-firefox-sidebar-access-bookmarks-history-synced If you want to hide the menu bar that shows in the screenshot, then you can hide this by right-clicking in empty on a toolbar and remove the tick on "Menu Bar" or via the toolbar at the bottom of the customize page. *https://support.mozilla.org/en-US/kb/restore-menu-bar-firefox *https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars