Mozilla was one of the last major browser developers to support Windows XP and Vista. However, that support eventually reached its hard limit:
Supporting a legacy operating system is a significant technical burden. Windows XP lacks the modern security architectures found in Windows 7, 8, and 10. It does not have the same Address Space Layout Randomization (ASLR) capabilities or advanced memory protection. As Firefox became more complex, ensuring it remained secure on a fundamentally insecure kernel became a nightmare for developers. firefox mozilla for windows xp
Chrome had become a memory-hungry titan, and Microsoft was preparing to pull the plug on Windows XP. As the "End of Life" warnings began to flicker like dying lightbulbs, Alex found himself at a crossroads. The modern web was becoming a fortress of heavy scripts and complex security protocols that his beloved OS could no longer bypass. Then, he found the fox. Mozilla was one of the last major browser
Today, running Windows XP on the internet is a security hazard. Using the last supported version of Firefox (version 52.9.0 ESR) is arguably the "safest" way to browse on XP, yet that is a low bar. The browser cannot render modern JavaScript-heavy websites, lacks support for current TLS encryption standards required by many sites, and contains unpatched vulnerabilities. It does not have the same Address Space
Mozilla became the last major vendor to maintain a modern browser engine for Windows XP. Long after Microsoft stopped updating IE8 (the highest version available on XP), and long after Chrome left the platform behind, Firefox continued to push updates. This was a critical service for the developing world, educational institutions, and enterprise environments where upgrading from XP was not a logistical possibility but a financial impossibility. Firefox provided a shield against malware and a rendering engine capable of handling HTML5 and CSS3, effectively prolonging the functional life of millions of XP machines.