Missing Windows 7 platform updates. Fix: Install KB4474419 (SHA-2 code signing support) and KB4490628 (Servicing stack update). Reboot, then run the Firefox installer again.
: If you have a newer, non-working version of Firefox installed, uninstall it via Control Panel > Programs and Features Run the Installer : Double-click the downloaded file (e.g., Firefox Setup 115.xx.x esr.exe Choose Setup Type for most users or if you wish to change the installation directory. Complete Installation to launch the browser. mozilla firefox download for windows 7 old version