In March next year, Windows XP and Vista users will automatically be moved to the Firefox Extended Support Release (ESR). Mozilla last week announced that they expect to provide security updates for users until September 2017. In mid-2017, Mozilla will reassess numbers on Windows XP and Vista and they will decide on the final support end date.
In the meantime, we strongly encourage our users to upgrade to a version of Windows that is supported by Microsoft. Unsupported operating systems receive no security updates, have known exploits, and are dangerous for you to use. For planning purposes, enterprises using Firefox should consider September 2017 as the support end date for Windows XP and Vista.
Please visit the Firefox support page for more details.