Okay, it's the "safe browsing" setting within Chrome then which does that kind of virus scanning of downloads.
Maybe there was an error in that SafeBrowsing data (whereever at google it will post the file to check it for virus), and maybe Firefox relies on the same safe browsing virus scanner
Or it even was esoui providing some info "wrong" but my downloads worked flawlessly.
Could have been so many things actually. If it happens again try to turn of the Safe Browsing setting at the browser settings and test if this fixes it (for that moment, turn it on again afterwards!).