Why?

Its not yours. Don't use it.

Why do you think its OK?

Why do you think that they are taking value away from Metalab by using the design as a mock-up that is never intended to be a final product?

I don't think that's the issue. For me its very simple: this is Metalab's Intellectual Property, and it should be respected.

In my opinion, it will be difficult if not impossible for Mozilla to create a truly original design if they are mocking up with Metalab's work.

Agree to disagree.

I didn't say I thought it was okay. You made a statement, I was asking for some explanation.