@gwen-dragon
My Chrome is Version 70.0.3538.102 (Official Build) (64-bit) and works fine
My Firefox is 63.0.3 (64-bit) and works fine
My Vivaldi has Chrome engine Chrome/71.0.3578.45 Safari/537.36 and does not Full Screen Video streams for NFL, but does work on Youtube.
Suspect a combination of the particular Chrome and the particular video player they use at NFL.