https://support.mozilla.org/en-US/kb/end-support-windows-xp-and-vista The now old vulnerable Firefox '''52.9.0esr''' (released back on June 26, 2018) was the last to still support the already EOL Windows XP and Vista. Note only the 32-bit version will run even if you have 64-bit WinXP as the Win64 version of Firefox required Windows 7 or later. Though Fx 116.0 or later now requires Win 10/11 to run. Here is the 32-bit Firefox Setup 52.9.0esr.exe https://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win32/en-US/ https://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win32/ You could also use the portable Firefox 52.9.0esr as the launcher keeps things self contained and can even run off a usb flash drive. https://portableapps.com/apps/internet/firefox-portable-legacy-52 or https://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20P.E.%20Legacy/Mozilla%20Firefox%2C%20Portable%20Edition%20Legacy52%2052.9.0/