The company has a plan to migrate the Windows XP and Vista users to Extended support Release in the month of March 2017 and later in September 2017 it will stop providing security updates to the old operating systems.
Even though anybody can use Firefox on their old operating systems, but they can’t get important security updates. Nowadays web attacks are major issue for the online users.
Without any security updates life becomes hell in the internet world. So it’s better to upgrade to the newer Windows versions such as Windows 10 to get important security patches and support for all the needed software including browsers like Google Chrome and Firefox.
Still there are a limited set of users for a decade old Windows XP operating system. This is the time to move on to new technologies. Already all other popular web browsers stopped supporting Windows XP and Vista. Firefox is only single browser that is providing support to the XP.