Can a Firefox window be re-opened if another Firefox window was open at closing time?
Solution 1:
Ctrl + Shift + T reopens the last "tab" or set of tabs. If you closed a window rather than a tab it will open the last window.
To reopen windows there is Ctrl + Shift + N
Alternatively there is the Recently Closed Windows area in the history:
If you click the > next to Recently Closed Windows then you should be able to reopen any recent window you closed.