alexarkadin wrote:
If the estate had been able to prove Kirby created characters at home and brought them to Lee then the case would have mirrored the Superman case where it was clear certain aspects of the character had been created before the copyright was sold to DC.

Not so.  The key distinction is that Siegel and Shuster could prove that they created the character before they were working for DC, and that they did not create it at DC's request or with the expectation that it would be published by DC.  They could show they attempted to sell the character to other publishers before DC bought it.  That made it not work for hire.  

Whereas at the time the FF was created, Kirby was working exclusively for Marvel.  Even if his estate could prove that he created the characters entirely by himself, they would also have to prove that he did so independently without any indication from Stan that Marvel wanted new superhero characters or was interested in publishing a superhero book.  They would also have to prove that Kirby created the characters without any expectation that Marvel would publish his work.  Absent proof of those things, the work is still considered for hire.  And IIRC Kirby's own testimony contradicts that scenario... didn't he say that Stan wanted to do a superhero book with old Timely characters, but that he came up with the idea of creating new characters?  That alone suggests the creation was done at Marvel's instance, even if Kirby did all the creating himself.

The part of Stan's testimony that is key is that he says he gave Kirby the assignment to do the book, not that he says he created everything himself.  The issue of who created what is not important to the judge's decision about whether it was work for hire.