Easy. Use PowerShell:
Invoke-WebRequest -Uri "https://download.mozilla.org/?product=firefox-latest-ssl&os=win64&lang=en-US" -OutFile "$env:TEMP\FirefoxSetup.exe"; Start-Process -FilePath "$env:TEMP\FirefoxSetup.exe" -ArgumentList "/S" -Wait; Remove-Item "$env:TEMP\FirefoxSetup.exe"