Firefox 52 will be the last COMPLETE version for Windows XP & Windows Vista.
Firefox security updates for XP and Vista users will continue until September 2017, although new features will not be offered.
http://news.thewindowsclub.com/firefox-52-xp-vista-87770/
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Remark: Firefox was the last "major" browser to support XP/Vista --- IE ( 8 ), Google Chrome and PaleMoon have all dropped support for XP.
As of the moment, I believe Avast's SafeZone browser still supports XP... but one has to wonder how much longer this will be.
We also have to wonder whether this will be the "final nail in the coffin" for XP...