How do I remove empty gray bar below the toolbar?

If the Bookmarks Toolbar is empty then Firefox would normally display a placeholder text, so I'm not sure why this area is empty for you. *bookmarks-toolbar-empty-message = For quick access, place your bookmarks here on the bookmarks toolbar. Start Firefox in <b>Safe Mode</b> to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if <b>hardware acceleration</b> or <b>userChrome.css</b> is causing the problem. *switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes *do NOT click the "Refresh Firefox" button on the Safe Mode start window *https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems