I was just told by a Ford engineer that the firing order only changed for the trucks in 2015 and it is now the same as the 4.6/5.4. The firing order for the mustang remains the same according to HP tuners' 2015 file.
firing orders:
15486372 - 4.6/5.4 and 2015 truck
13726548 - 5.0 and 2011-2014 truck