Basically this means that the processor in your computer is too old for Firefox to run on.

https://support.mozilla.org/kb/your-hardware-no-longer-supported

Because you have such ancient hardware, you are running an ancient operating system. You need to purchase a new computer to stay safe on the web. It is no longer safe to use Windows XP on the internet.

If you don't have SSE2, the last version available for you is 48.0

100 years ago we've dropped support for old processors.

Try to install (link removed by mod), then upgrade to the latest available 48.* using Help -> About Firefox .

Since your CPU does not support SSE2, this means for Windows Firefox 48.0.2 Release and the much more current 45.9.0 ESR were the last as 49.0 and later on Windows required CPU's that supported SSE2.

http://archive.mozilla.org/pub/firefox/releases/45.9.0esr/win32/ and pick your locale.

But really you should not be using your old WinXP system online since any web browsers you can run on that system is rather old and vulnerable now.

The only way to use a newer version of Firefox on your old hardware is if you use a light Linux distro that still supports the old 32-bit CPU's (Lubuntu or Xubuntu or even lighter distro options) as unlike Windows (dropping at 49.0), Firefox on Linux as of 53.0 and later dropped support for the old CPU's that did not support SSE2. This means you can use the Firefox 52 ESR that had last major update 52.9.0esr out on June 26. However the legacy 52 ESR will be EOL on Sept 5 with no more updates then.

Thank you James.