I can't think of a built-in setting for that exact behavior. The closest thing that comes to mind is the setting to generate a new tab when you close the last tab in the window. This forces you to close a window using the X button, but when it's restored, you should just get the new tab: 

For other session restore problems, you may need to make corrections to your Firefox settings: # Click the menu button. ... # Select the Privacy & Security panel. # In the History section, make sure that the Firefox will drop-down menu is set to either Remember history or Use custom settings for history. OR.... I can't think of a built-in setting for that exact behavior. The closest thing that comes to mind is the setting to generate a new tab when you close the last tab in the window. This forces you to close a window using the X button, but when it's restored, you should just get the new tab: (1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk. (2) In the search box above the list, type or paste LASTTAB and pause while the list is filtered (3) Double-click the browser.tabs.closeWindowWithLastTab preference to switch the value from false to true true to false