When I installed Firefox 3b5 on Windows Vista, I could no longer download files because they were marked as possible malware. The download window would show the file with the message, “Blocked: Download may contain a virus or spyware.” I tried changing the security settings, as suggested, to no avail. I gave up on it and figure it must be a bug.
Now it works. The change was simple. I ran Windows Update. I discovered Windows had not updated, contrary to what I thought, for a few months because some Windows service had been turned off. Once I got Windows Updated working again with a restart or two, Firefox 3b5 now can download files again.