For Mozilla's sake, I hope it was the design team they hired that did the knockoff rather than an inhouse designer. Although it's probably too early to jump to conclusions at this point.
If they did, how would they know which site to knock off? Unless it's either a bizarre coincidence, the Mozilla guys would have had to pass the mockups on to them. And they still had to upload them afterwards!
I'm sure there were more than a few designers who tendered an offer.
But a strange coincidence that they just happened to pick Metalab's design to rip off, no?