The problem here is to weight impact against implementation costs (even if this is completely done by volunteers, there are still opportunity costs). Maintaining an antivirus database is something that pretty big companies specialized on and are investing all their time. Further I think that protecting Firefox users is important, nevertheless we have to choose our battles. We should focus on making Firefox and our other products as good as they can be, instead of jumping into another (really big) topic and keep focusing on browser-relevant security such as SSL/TLS, safe browsing and addon-on security reviews.
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