Mozilla announced plans to support Firefox for Windows XP & Vista until at least September 2017. All XP and Vista users will be moved to Firefox ESR (Extended Support Release) ESRs are Firefox versions for which Firefox provides security updates for extended amounts of time. Mozilla will assess the browser’s status on these operating systems in mid-2017, and decide if they’ll continue to provide support after September 2017, the browser maker said. Google announced end of support for Chrome on Windows XP, Windows Vista, OS X 10.6, 10.7, and 10.8 in April 2016.
Source: https://www.bleepingcomputer.com/news/software/mozilla-to-support-firefox-for-windows-xp-and-vista-until-september-2017/