Firefox 52 is out today, and it’s a landmark release for a couple of reasons.
The release is the final major version to support two legacy operating systems: Windows XP and Windows Vista. Future major versions of the browser will require at a minimum Windows 7. Firefox 52 is an Extended Support Release; it will receive security fixes (and only security fixes) for approximately one year. New features, however, will be restricted to the mainline version of Firefox. Microsoft no longer supports Windows XP at all, and Windows Vista drops out of extended support on April 11, 2017. Google dropped Windows XP and Windows Vista support in Chrome in April 2016.
As such, users of those operating systems will still have an actively patched browser for a little while longer, but their days are numbered.