In a post on Mozilla’s blog, the developer revealed that they will be supporting Firefox until September 2017 at least. What this means is that you can still expect updates to the browser despite the fact that your operating systems are no longer getting mainstream support from Microsoft.
According to Mozilla, “Firefox is one of the few browsers that continues to support Windows XP and Vista, and we expect to continue to provide security updates for users until September 2017. Users do not need to take additional action to receive those updates. In mid-2017, user numbers on Windows XP and Vista will be reassessed and a final support end date will be announced.”
Note that it says until September 2017 after which the numbers will be reassessed, which means that should the numbers show that there are still quite a few users on those platforms, Mozilla could further extend support, although the developer is encouraging users to upgrade if they haven’t done so already.