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What's worrisome is that if they were willing to burn this Firefox JavaScript exploit, then that probably means they know of at least one more.

Keep in mind that this was a known and fixed vulnerability, not an 0day. 'Burning' is a bit misleading in this context.

0days like this go for $80k to $200k each for exclusive rights. The FBI owns dozens, at least.

Source: HBGary dumps, industry experience.