Are you getting a notification from Firefox on the drop-down from the Downloads button? That is from the phishing and malware protection feature. More info in this article if you want to try turning off the "Block dangerous downloads" feature temporarily:

How does built-in Phishing and Malware Protection work?

These were already set (unchecked)

What indication are you receiving that Firefox is blocking the download?

These are the messages I get.

I think it's Windows Defender, but I haven't studied threads like the following in detail:

https://answers.microsoft.com/en-us/windows/forum/all/unable-to-add-exclusion-in-windows-security/4c8462b7-b781-43d2-8e30-ef233d23d76d

Yep, the Real Time Protection was the culprit. Thanks for your assistance. LLAP