It seems pretty straightforward. Trond Hansen was apparently paid to do some consultancy design work for Opera. After accepting his paycheck and moving on, he decided to take the design(s) he made while at Opera, and give it to Mozilla.
This doesn't sound like a very clever thing to do. It sounds very illegal indeed.
Depends on the contract he signed, and where he signed it. For example, some contracts stipulate all work inside and outside of work hours belongs to the employer, most others don't.
This doesn't sound like a very clever thing to do. It sounds very illegal indeed.