Press the Alt or F10 key to bring up the tool bar. Then select History. Look at the top of the menu.

If the window that had that tab is still open and in focus, you can:

• press Ctrl+Shift+t
• right-click any other tab > Undo Close Tab
• from the History menu, check the Recently Closed Tabs list (it's probably the top item)

If the window is completely closed:

• from the History menu, check the Recently Closed Windows list (it's probably the top item)
• If it was a private window, sorry!

Warning: If you use the History menu from the "3-bar" menu button, watch out for "Restore Closed Tabs" because that will re-open everything listed below it. To open just one, click the specific one.

See "History > Recently Closed Tabs/Windows" or use the Shift+Ctrl+T shortcut key.

You can find history on the menu bar (tab the alt key or press F10) on in the "3-bar" Firefox menu button drop-down list.