You can download old Firefox version from https://ftp.mozilla.org/pub/firefox/releases/

The last version for Windows XP was the Extended Support Release (ESR) version 52.9.0. https://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win32/ choose your language

for Windows 7 Extended Support Release (ESR) version 115 https://www.mozilla.org/en-US/firefox/all/desktop-esr/win/en-US/

Windows 7, 8, 8.1 is still being supported with the old Firefox 115 ESR channel currently at 115.28.0esr and will be updated till Fx 115.33.0esr in February 2026 unless Mozilla does a fourth updates extension. It is basically based on the Firefox 115.0 release but with security/stability fixes since.

You should not use Windows XP or a old Firefox version like FX 52.9.0esr online.