Microsoft stopped support for windows XP a couple of year ago, after which most third party app developers followed suit. A few like Firefox still support the windows till today but that will be ending in 2017.
According to a blog post on Firefox’s website, It will not only stop supporting windows XP: Windows Vista will also stop receiving support.
The process of stopping this support will start by March 2017 when Firefox will first move users of the two windows to Firefox Extended Support Release–a version of Firefox with limited features and supports. And by September the support will finally end.
Stopping support doesn’t mean Firefox wouldn’t work on Windows XP and Vista again
Stopping supports for these two OS doesn’t mean users wouldn’t be able to run Mozilla Firefox on them again. It means the users wouldn’t be able to run the recent release of the browser. Also nobody would support them in the case of security flaws and hacks.
So to continue enjoying the supports and features of recent release of Firefox, the app developer advises users using XP and Vista to upgrade to version of windows supported by Microsoft.