If you must use Firefox on XP Professional, follow these steps to install the final compatible version:
As of March 14, 2025, an expired root certificate has caused widespread functionality loss, making many modern websites inaccessible without manual certificate updates. 🛠️ Installation Guide for Windows XP Firefox Windows Xp Professional
End-of-Life. No security patches or feature updates have been issued since 2018. If you must use Firefox on XP Professional,
Firefox 52.9.0 ESR (released June 2018). Firefox Windows Xp Professional