I don't see a threat to sue here? I see him saying he will send Mozilla an invoice for $18,000 based on his (very rough, very inaccurate, very arbitrary) estimate, but there's no legal force behind that. It clearly was an idle threat and his last responses seem to indicate he was resigned to having to eat his labor and treat the cost as a lesson learned.

But the title of this post is a bit inaccurate in that he didn't threaten to sue. He threatened to send Mozilla an invoice for his work, but he came out looking very foolish, and ended up tucking tail.