SOLVED with sending ENTER command to save the file using SendKeys in PowerShell script:
XRun | pwsh.exe | -NoProfile "C:\MyPath\powershellscript.ps1"
In PowerShell only 2 lines of code:
Add-Type -AssemblyName System.Windows.Forms
[System.Windows.Forms.SendKeys]::SendWait("{ENTER}")
In the FireFox SaveAs window you can change the filename but commands such as TAB, ENTER, ALT+S are blocked from XType.