Even though all popular web browsers such as Chrome and Microsoft’s own Internet Explorer have discontinued support for Windows XP and Vista, Firefox has continued to support them till this date. However, this support is going to end soon given the outdatedness of the two OS. Microsoft already ended support for XP in 2014 and will end support for Vista in 2017.
In March 2017, Mozilla will move existing Firefox users on both OS to Extended Support Release version. The last updated version will be Firefox 52. No new features will be added in the updates after that, but users will still receive Security updates till September 2017. The final support end date will be announced in mid-2017 based on number of users still on Windows XP and Vista.
For existing users, to keep up with new version of Firefox they can either opt to upgrade their OS to Windows 10 or 8 or 7, or if their system is too old to support newer version of Windows they can switch to Linux based OSes like Ubuntu and Fedora.
Link : Mozilla’s Support website.