In approximately March, 2017, Windows XP and Vista users will automatically be moved to the Firefox Extended Support Release (ESR).
v52 will be the last for XP/Vista, 6 weeks later v53 is not for it. so XP/Vista systems (with -paid- current patches) should change to v52 esr and THAT is maintained up to september. the reality is much more worse.
Pale Moon also will not be supported as dev dropped XP support back in November 2016 with the release of version 27. I also noticed SeaMonkey 2.49 will be the last version supporting Windows XP/Server 2003 and Vista/Server 2008. Note: SeaMonkey 2.46 was just recently released. Don't know if "KernelEx" community will keep Firefox running on XP/Vista.