The last working version of Firefox for Windows XP is Firefox 52 ESR. It can be downloaded from ftp(dot)mozilla(dot)org/pub/firefox/releases/52.9.0esr/

That version is severely out of date, as is the Windows XP operating system. For context, we are currently at Firefox 77 and Firefox 52 ESR has not been updated since fall of 2018. By using an older version of Firefox you will be exposed to various unpatched security flaws. USE AT YOUR OWN RISK.

NOTE: Because links cannot be posted in this forum, I was forced to place any dots in the link with (dot).

http://ftp.mozilla.org/pub/firefox/releases/52.9.0esr/win32/

Besides using Windows 7 or later you could dual boot with a Linux distro with a lighter desktop environment like Xfce in order to use current versions of Firefox.

Also with WinXP you do not fully support HTML5 players as Microsoft only provides media feature packs to Vista and later. Note if you do use Linux make sure you install packages like FFmpgeg to full support HTML5.

Adobe is discontinuing support of their Flash Player Plugins by end of 2020 and very few sites are still making use of it and using HTML5 players on site instead for video, audio, games etc.