Because it is a legacy version, you cannot download it through the standard "Download" button on the main Mozilla website. Instead, you must access the official Mozilla archives: Firefox ESR 52.9.0 System Requirements
Finding a compatible browser for Windows Vista is challenging since most modern software has dropped support for legacy operating systems. If you need to download Mozilla Firefox for Windows Vista, you must use a specific older version, as current releases only support Windows 7 and newer. The Last Compatible Version: Firefox 52.9.0 ESR
The final version of Mozilla Firefox that officially supports Windows Vista is . Released in June 2018, this was the terminal update for the Vista and XP platforms.
While it is no longer updated with security patches and is considered unsafe for general web browsing, it remains the most functional official version of Firefox for legacy hardware. Where to Download Firefox for Windows Vista