The tags show Linux erroneously as I made this post from Linux. It's about Firefox on Windows of course. Can someone with sufficient rights fix the tags? It's about Windows and the latest FF Version.

I think Defender should scan all files as they are written, but maybe the application needs to trigger that? As you probably noticed, Firefox creates a temporary placeholder and the actual download is first saved as a .part file. Can you tell whether Defender is scanning the .part file?

I removed the Linux tag; can you add your Windows version using the Tags box to the right of the question (type Win to generate matches)?

All security software is supposed to block the EICAR test file, so you can use that to see whether it survives the on-download scan:

https://www.eicar.org/download-anti-malware-testfile/

I get a zero byte file and a yellow triangle on the Downloads button (indicates the download failed or was blocked), but I'm not using Defender.

There is a Reddit Post regarding this issue https://www.reddit.com/r/firefox/comments/yk5qne/windows_defender_not_scanning_files_that_firefox/

I removed the Linux tag; can you add your Windows version using the Tags box to the right of the question (type Win to generate matches)?

I can't find a way to add tags. Tried edit this question.

Not working

Not working

Are you confirming the issue or what are you refering to with "not working".

I added a tag for "Windows 10"

I can't test Defender due to having other security software (IT department managed).