If you cancel a download in Safari, or it errors out, and then you clear the downloads list, the partial file is deleted. This makes sense because you no longer are able to retry that specific download.
Firefox deletes the partial downloads as soon as the download is canceled - does not wait to delete it.
Orion does not clear partial downloads, even when the download list is cleared.
I suggest following Safari's behaviour here, using file hashes + filename to ensure the correct file is deleted when the list is cleared