The “On Fire” morale status is greater than “Superb” and it can be triggered only on special occasions. Having a player winning the Top Scorer award in any Top Eleven competition or lifting a prestigious trophy are just some ways to make your player reach the "On Fire" status.
Keep an eye on the morale feedback in the players' profiles to see the situations that led your player to achieving the “On Fire” morale status. Note that the “On fire” morale status can only be triggered with certain events, since this state cannot be reached by simply using morale boosters with your players.