Because Microsoft ended support for Windows XP in 2014 and Mozilla ended support for this version in 2018, using Firefox on XP today presents significant security risks. Final Version : Firefox 52.9.0 ESR.

Official downloads are no longer available on the main Firefox homepage, but can be found via Mozilla’s Archive: Navigate to the Mozilla FTP/Archive. Select the or English (GB) folder. Download the file named Firefox Setup 52.9.0esr.exe . Run the installer on your XP machine. Current Limitations & Security Risks Firefox new for an XP laptop - Mozilla Support

The latest and final version of that officially supports Windows XP (English) is Firefox 52.9.0 ESR (Extended Support Release), which was released on June 26, 2018.

: Only the 32-bit version is compatible with Windows XP; the 64-bit version of Firefox requires Windows 7 or later.

: You must have at least Windows XP SP2 or SP3 installed to run this version.