Why when I set "Firefox will: Never remember history", Firefox still remembers it in "Recently Closed Tabs/Windows"? I don't want to restart my browser

If you set "Firefox will: Never remember history" then you enter Private Browsing mode immediately. You can see a grayed "Tools > Stop Private Browsing" to confirm that. This doesn't affect already stored History, but Firefox won't add new visited links to the History. If you want to remove sites from the History then you need to do that yourself. You can (temporarily) set the pref [http://kb.mozillazine.org/browser.sessionstore.max_tabs_undo browser.sessionstore.max_tabs_undo] to ''0'' to clear the ''Recently Closed Tabs'' list. Use the ''Reset'' context menu item to re-enable ''Undo Closed Tabs'' with the default value if you want to re-use that feature. You can temporarily set the pref browser.sessionstore.max_windows_undo to 0 to reset the Recently Closed Windows list, However there appears to be always one window left that is only removed if you close and restart Firefox. You can avoid that by closing all tabs before closing the window with only an empty tab left. Set the pref '''browser.tabs.closeWindowWithLastTab''' to ''false'' on the about:config page to prevent closing the last tab from closing the browser. See http://kb.mozillazine.org/about:config