744 SOs often upgrade onto the Airbus, as the 777 is only a relatively small fleet in terms of crewing.
Most 777 JFOs have come from the 744, but every now and then you do get an Airbus SO upgrading onto the 777.
of course, now we are about to have SOs on the 777 who will also fly the 744 as SO, and these people will become JFO on the 777.