Mozilla plans to halt all Firefox security updates next year to personal computers powered by either Windows XP or Windows Vista, closing that chapter of its browser’s history.
Windows XP and Vista have both been retired by Microsoft, XP in April 2014 and Vista in April 2017. Once dropped from support, an operating system stops receiving security updates from Microsoft. Third-party application developers, however, may continue to support their products on retired versions of Windows.
Source: Mozilla to stop all support for Firefox on Windows XP and Vista | Computerworld