Have you tried hitting Ctrl+Shift+N to restore the previously closed windows? These will behave differently: (1) "Quitting" Firefox will multiple windows open.

Have you tried hitting Ctrl+Shift+N to restore the previously closed windows? These will behave differently: (1) "Quitting" Firefox will multiple windows open. (2) Using your WM to close both Firefox windows. The latter will cause only the last-closed window to be restored, but you can pop the others off the stack too with Ctrl+Shift+N.