Mozilla plans to end mainline support for the Microsoft operating systems Windows XP and Windows Vista with the release of Firefox 53 in March 2017.
This means that Firefox 52 will be the last feature update for those operating systems, as Firefox 53 cannot be installed anymore on those operating systems.
Mozilla explicitly mentions installations that it plans to block. It is unclear whether it plans to block execution of portable versions of Firefox as well.
The organization plans to migrate Firefox users on XP or Vista to Firefox ESR automatically however to extend support.
Firefox 53: no support for Windows XP or Vista
Tip: load about: in Firefox's address bar to display the version of the browser installed on your machine.
Bug 1305453 on Bugzilla lists the plan to stop stand-alone Firefox 53 and up installers from installing Firefox on XP or Vista machines.
We plan to eol XP/Vista by first moving those users out to ESR 52. Once 52 merges to aurora, we should land changes to the stand alone installer to prevent install by XP and Vista users. Initially there shouldn't be an issue with running but eventually we'll import a system dependency that will break browser startup.
Another bug, bug 1303827, highlights Mozilla's plan to move XP users to Firefox's ESR branch when version 52 gets released. Firefox 51 is therefore the last version of the browser that is not ESR.
We've decided to move Windows XP to the Firefox 52 ESR branch, so that Firefox 51 will be the last mainline version to support the platform.
We still don't have an official EOL date for XP support, but moving the platform to the ESR branch means that we will *not* need to worry about new feature support for XP which is becoming increasingly difficult.
It is unclear as of now for how long XP or Vista will be supported on the ESR channel. Firefox ESR 52 will be supported for quite a while. The new version will become available with the release of Firefox 52 on March 7, 2017, and will be supported until mid 2018.
Firefox users on XP or Vista may use Firefox therefore at least up until that time unless Mozilla decides to remove support for those operating systems from ESR at one point before the release of Firefox ESR 59 (which seems unlikely).
Microsoft ended support for Windows XP on April 2014, and plans to end support for Windows Vista on April 2017. Windows XP has not received security updates since April 2014, unless you have applied this little tweak to the machine. Google stopped supporting XP and Vista back in Chrome in April 2016 already.
Mozilla Firefox is the last major web browser that supports XP and Vista (when you look at use figures). (via Sören Hentzschel)
Now You: are you affected by the decision?