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