It's a current mockup of a UI that appears to be copy-pasted directly from the website of a UI design company that put in a bid on this very project and was rejected.
At best, this was a gross and irresponsible move by Mozilla. And if it's not planned to be in the final product, it's also misleading.
It's still irresponsible to use those images and that layout, after rejecting a bid by that design company to create images and layout for the product, even in a mockup.
It is irresponsible, for sure. But is an appropriate reaction to set up a big witch hunt and publicly shame Mozilla? Is that how businesses and organizations should be treating each other? People make mistakes. Give them a chance to make things right before you try to do damage to someone's reputation.
Mozilla also recorded and released a lengthy video tutorial portraying and promoting the product, which entirely uses MetaLab's design, with absolutely no indication that the design is only an internal mock-up.
(They have since made the video "private" on Vimeo, though it is still on all of their promotional material.)
At best, this was a gross and irresponsible move by Mozilla. And if it's not planned to be in the final product, it's also misleading.