In approximately March, 2017, Windows XP and Vista users will automatically be moved to the Firefox Extended Support Release (ESR).

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...
Source: https://blog.mozilla.org/futurerelea...-xp-and-vista/

See also: https://support.mozilla.org/en-US/kb...s-xp-and-vista