As of Firefox 53 in March 2017, WinXP and Vista won't be supported any longer. http://www.ghacks.net/2016/09/27/firefox-53-no-support-for-windows-xp-or-vista/

https://support.mozilla.org/en-US/kb/end-support-windows-xp-and-vista?cache=no

https://support.mozilla.org/t5/Install-and-Update/Important-Firefox-is-ending-support-for-Windows-XP-and-Vista/ta-p/31270

What about support for Firefox ESR (extended support release)? The support article Important - Firefox is ending support for Windows XP and Vista still says:

"...Firefox security updates for XP and Vista users will continue until September 2017, although new features will not be offered. In mid-2017, a final support end date will be announced based on the number of users still on Windows XP and Vista..."

I received the 28-Sep-2017 update to FF ESR v52.4.0 but I understand major changes are coming for FF 57 in November 2017 when the new Quantum engine goes live. Does Mozilla have any plans to pull the plug on XP/Vista before FF 57 / FF ESR 52.5 is released on 14-Nov-2017?

32-bit Vista Home Premium SP2 * Firefox ESR v52.4.0

Hi XPFox:

Mozilla posted an official announcement yesterday (04-Oct-2017) on their Future Releases blog at Update on Firefox support for Windows XP and Vista, which states in part "Today we are announcing June 2018 as the final end of life date for Firefox support on Windows XP and Vista."

That means that Firefox ESR v52.8.x (scheduled for release on 01-May-2018) will be the last ESR security update that is compatible with Win XP and Vista.

32-bit Vista Home Premium SP2 * Firefox ESR v52.4.0