Starting in the nightly build 2010-02-02 I've had the download manager never stop "Scanning for viruses...". I don't think I have a virus scanner installed in any case (but I'm not sure). This seems similar to bug 450700, but it just started so I'm filing it as a new regression. I don't see anything in my error console, but it may have just scrolled out since I have a zillion tabs open. Happy to provide more information if that would be helpful.