I hope they reach an agreement where Metalab is reimbursed for the entire amount of their initial quote.
Why on earth would/should that happen over a few mockups? It's not that Mozilla asked Metalab to do a bunch of work, claimed it sucked so they wouldn't pay, and then used it anyway. No: Moz decided not to hire Metalab, and someone ripped off some existing work of Metalab's for some mockups.
Now, I think that was an inexcusable thing to do on the part of whoever did it, but even if it was actual Mozilla staff, I just don't see why Mozilla ought to pay Metalab for not working nor hire them unless they happened to be the second choice. (Because the first choice--assuming they were responsible--has got to go.)
(By which I mean that a settlement offer along the lines suggested is part of how to avoid the kind of tangle you describe.)