Are there still people using Windows XP and Vista? I know of most cyber cafés who have vowed never to upgrade to the latest windows… Mozilla got bad news for you.
"Firefox is one of the few browsers that continues to support Windows XP and Vista, and we expect to continue to provide security updates for users until September 2017. Users do not need to take additional action to receive those updates. In mid-2017, user numbers on Windows XP and Vista will be reassessed and a final support end date will be announced”
Meaning, from September 2017, Firefox will end supports for Windows Vista and Windows XP hence, you are advised to upgrade before that date.