If you close an article while other tabs are still open, use Firefox’s menu on your computer and open History → Recently Closed Tabs. Choose the entry by its title. If you closed a separate window containing several tabs, look under Recently Closed Windows. That list is useful when a single tab would leave the rest of your work behind.
Check the window you wanted
Imagine one window contained a museum timetable and a route map. If you closed the whole window, look for its entry and check that both pages return. If you closed only the map, use the tab list and verify that the address matches the intended map.
When the entry is missing
Private windows are excluded from normal session recovery. If an ordinary page is no longer in the closed list, check browsing history for its title or address if history was retained. Reopening a page can bring back its URL. Check whether any unsent form text is still there before continuing.