Following Microsoft's announcement that it does not support Windows XP and Windows Vista, almost all browsers other than Firefox have followed its policy.
For example, Chrome no longer supports these two, but the Firefox has still included them in his upgrades. But recently, the Mozilla developer announced plans for them.
So it looks like Mozilla will stop supporting them in September 2017. However, around March 2017 Firefox will connect these older operating systems to the ESR (Extended Support Release) channel automatically.
Users running both old Windows and Firefox at the same time do not need to do anything. The change will happen automatically. Your browser will continue to receive updates.
Of course, to clarify, it is not clear when Mozilla plans to stop support for Windows XP and Windows Vista. Specifically they said:
In the middle of 2017, the number of users in Windows XP and Vista will be reassessed and a final end date of support will be announced.
So at least until September of 2017, you can enjoy Firefox if you can not or do not want to move for any reason into a newer operating system.