Hi love.arijit, Firefox's default setting is "Preview in Firefox" so it's very odd that it changes to Save File when you restart Windows. Does it also change when you simply Exit Firefox and start it up again without restarting the whole system? There are some utility/privacy/cleaning programs that can touch browser settings files, including Advanced System Care and CCleaner. If you have anything like that, please set the program(s) not to touch your Firefox data and see whether that makes any difference.