If you already know the direct FUS URL (e.g., from a friend’s log), you can bypass SamFirm entirely. However, this requires the decryption key.

Update certs:

While the original tool was designed exclusively for Windows and is now deprecated, several modern, cross-platform alternatives allow you to download and decrypt official Samsung firmware on Recommended Linux Alternatives

Running the Windows SamFirm tool

Your first firmware download is just a command away.