As of Firefox 53 in March 2017, WinXP and Vista won't be supported any longer. http://www.ghacks.net/2016/09/27/firefox-53-no-support-for-windows-xp-or-vista/

https://support.mozilla.org/en-US/kb/end-support-windows-xp-and-vista?cache=no

https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

Using another computer, Download Firefox Full Version For All Languages And Systems onto a thumb drive. After, plug the thumb into your system.

To save the OP the trouble on what and where to download from that will work on the old EOL WinXP the useful link is https://www.mozilla.org/firefox/organizations/all/

The link has the full setup and not the small online stub installer.

The Firefox 52 ESR will likely get security and allowed stability updates until Firefox 52.8.0esr out when Firefox 60.0 is Released in May 2018 unless Mozilla extends support of it.

You can use a usb flash driver or CD or even waste a DVD disc to transfer the Firefox setup to the WinXP computer.

Note if one misses a Firefox update then the next update will be a full update instead of a small partial. Not a problem for faster connection but for dial up it will be.