These days, most mainstream browsers do not support Windows XP and Windows Vista operating systems. Once Microsoft ended their official support, many vendors did the same. For example, Chrome doesn't support them, but Firefox still does. Mozilla today shared details of their plans for Windows XP and Windows Vista.
Firefox is the only mainstream browser left which still supports Windows XP and Windows Vista. Mozilla announced that they expect to release security updates for Firefox users till September 2017. However, in around March 2017, Firefox on these older operating systems will be switched to the ESR (Extended Support Release) channel automatically.
The good news is that users do not have to do anything. The switch will happen automatically. The browser will continue to receive updates.
It is not clear when exactly Mozilla plans to drop support for Windows XP and Windows Vista. They said:
In mid-2017, user numbers on Windows XP and Vista will be re-assessed and a final support end date will be announced.
So, at least till September 2017, you can enjoy Firefox in case you cannot move or do not wish to move to a newer operating system for whatever reason.
Mozilla is recommending users to consider upgrading to an officially supported version of Windows. For planning purposes, enterprises using Firefox on older systems should consider September 2017 as the support end date, according to them.