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By their own admission, Mozilla has released a plugin - Vietnamese language pack for Firefox 2 - which contains malicious code. The code is a result of a virus infection, but it's not a virus itself; still, it's malicious enough to make your Firefox displays unwanted ads (is there such a thing as wanted ads?), which is bad enough.
Window Snyder, chief of Firefox security, claims there have been 16,667 downloads of the Vietnamese language pack since November 2007; it's a lot or a little, depending how you look at it. Still, this is the kind of thing that makes Firefox security look bad, so please take care that it doesn't happen again, guys.