Mozilla joins Google, who dropped support for XP and Vista back at version 50, released in April 2016, along with other older platforms such as OS X 10.6 Snow Leopard, OS X 10.7 Lion, and OS X 10.8.Windows XP reached the end of its life on April 8, 2014 and is no longer receiving security updates from Microsoft. Vista on the other hand received security updates until April 2017, but that version of Windows never really gained traction so it's easy to see why companies are dropping support much sooner.
Firefox users that still need a browser for their old and antiquated XP and Vista machines would have to use old Firefox and Chrome versions, but both Mozilla and Google recommended that users upgrade to Windows 8.1 or newer, for security reasons.
Despite all security warnings, the latest statistics from NetMarketShare reveal 5.69 percent of users are still running Windows XP.