IIRC, Chrome does the same thing too. I think it's not much of an issue for Firefox to flag stuffs downloaded from suspected URLs as a malware since it's not uncommon to have one's system infected from those sites' content. Firefox is just trying it's best to prohibit any sort of system infection through itself.
In that case I would change the wording from "contains malware" to "may contain malware".
Also, they're prohibiting nothing. They still give you the option to open the file (which, as explained by the article, opens a whole different, albeit small, can of worms)
Or even more precise: "may contain copyright infringement".
Their (and chrome's) current solution to block malware domains use a client-side bloom filter afaik.
If you'd try to build the same client-side into firefox, you'd have just built another (bad) antivirus software.
Might as well integrate clamav into firefox then.
* Privacy
* Performance
* They would likely have to work with Virus Total to support their infrastructure as I can only imagine how quickly they'd take such a cloud service offline if everyone started using it by default
* And then what happens if / when the cloud service does have an outage? Does that mean people are blocked from downloading things?
* Same question for people on a corporate network who might have Virus Total blocked
* Same question for people on poorer internet connections as now the user has to transfer twice as much data if it's not a hash already stored on Virus Total.
That all said, its a cool idea for a third party browser add-on (if it hasn't already been done?)
_Privacy: They [FF] can act as anonymous proxy.
_Perf: Yes, for new objects, but for known objects, it should be minimal.
_Service outage: Build a system which can allow an override (download at your own peril, while service is out)
_Corp should already have enterprisey systems in place
_New, unknown object: Yes an issue.
As someone else said [GlitchMr], if they can do it ala haveibeenpowned I think it's worth looking into.
I think he's talking about the file contents.
Wait, why am I not using Chrome then in the first place?