The sidebar would normally stay closed if you close is and close and restart Firefox. Isn't that happening to you ? Can you attach a screenshot? *https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem *use a compressed image type like PNG or JPG to save the screenshot Start Firefox in <b>Troubleshoot 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 if <b>userChrome.css/userContent.css</b> is causing the problem. *switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes *do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window *https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Also, Firefox has a "semi-retired" feature to reduce the heights of the toolbars. See: [[Compact mode workaround in Firefox]].