The key here is not that M$ may be off a $521 million hook. No, it is simply that the patent office invalidated the Eolas patent (also known as the 906 patent):

A system allowing a user of a browser program on a computer connected to an open distributed hypermedia system to access and execute an embedded program object.

This patent should not have been awarded in the first place. Not only is there substantial prior art but this type of software process patent should not qualify for a patent. Yea, I know the patent office has been granting these types of patents but, come on, let’s restrict patents to things that take, say, a bit of originality and genius. Not things that your average programmer or system analyst knocks out routinely.
Via mozillaZine.