What a crap article: first, the malware has nothing to with a Firefox exploit. It is just something that infects a PC by any other means (the article doesn't say which), just like any other malware would.
Then the article describes the unwanted HTML in a language pack, which is totally unrelated. I guess they want to suggest that the malware in question could slip into Firefox in the same way, but there is no reason whatsoever to assume that would be the case.
It's not a bug at all, but then, no one said it was.
It's just a new type of malware: a firefox addon based malware. How it gets installed is a different question, but the takeway from the article is that it's now necessary to virus scan addons.
Then the article describes the unwanted HTML in a language pack, which is totally unrelated. I guess they want to suggest that the malware in question could slip into Firefox in the same way, but there is no reason whatsoever to assume that would be the case.