F only. The J footstool isn't strong enough to sit on, especially NCW 2.0, it appears, to start with.
On a trip back from BOS last year, though, the guy in the same row as me in Y had his missus in J - he disappeared for a 'visit', never to be seen again. This was the day flight, so it wasn't full. Also on this flight, a guy sitting at 26F next to a mum, child & infant offered to move somewhere else so the family could have more room - and the crew accepted his kind offer and put him in W!