I use custom history settings. I uncheck Accept cookies from sites (and then use the Cookie Monster add-on to manage cookies). Occasionally I find that the preference has returned to Remember History. I can detect no pattern to when this happens. It usually happens during a session (ie I have not shut Firefox down). It is possible that it happens after the computer has been in sleep mode, but it certain does not happen whenever it is put in sleep mode. Sometimes there may be a couple of weeks between incidents. As I only become aware of it by paying close attention to what Cookie Monster is reporting as the default cookie setting, I am never sure exactly how long it has been before I notice it, though I try to pay attention nowadays.