I do not want Firefox to restore my last session unless there was a crash, and thus I have the setting "Open previous windows and tabs" unchecked. However, if I reboot my PC and reopen Firefox, it brings up my last set of tabs. And I always close the entire Firefox window before shutting down, so there's no way it should think that it had crashed. This doesn't seem to happen when I close Firefox and reopen within the same PC session (i.e. without shutting down in between).