Windows XP and Windows Vista will no longer be supported starting with Firefox 53, scheduled for March 2017. After that date none of the popular browsers provide an up-to-date browser for both outdated operating systems.
Users on XP and Vista can still use the Extended Support Release (ESR) of Firefox 52 for which only security updates will be released till mid-2018. The decision to stop support for the older operating system is made because it saves developers time. They no longer have to support new features on the outdated operating systems.
Non-ESR Firefox versions will continue to work with XP and Vista but as soon as new features are added that require new Windows system dependencies, the non-ESR versions won't work anymore. Mozilla will then also make changes to the installer so the browser can't be installed on XP and Vista systems anymore.
The end of support comes a month before Microsoft ends the official support for Windows Vista, on April 11th 2017. Earlier this year also Google Chrome stopped supporting XP and Vista.
Chrome started to show a notification warning users that support for their OS ended, it's unclear whether Firefox will also alert users on XP and Vista that the end is near.
