Brett McLean added two power-play goals for Florida, which matched a season high with its fourth straight win. Both of McLeans goals bounced in off Atlanta players.
Bryan Little and Ilya Kovalchuk scored power-play goals for Atlanta, which snapped a nine-game losing streak Friday with a shootout victory over Minnesota.
Kovalchuk has scored in all five games this season against the Panthers.
Florida went back to All-Star goalie Tomas Vokoun after backup Craig Anderson played three straight games, including recording back-to-back shutouts on the road. Vokoun finished with 25 saves after giving up a goal on his first shot.
Atlantas Kari Lehtonen stopped 38 shots.
Horton scored during 4-on-3 play during overtime when he put a slap shot from the right circle over Lehtonens left shoulder.
Florida, which rallied from a 2-0 deficit, dominated overtime with a 6-1 shots advantage.
Little opened the scoring 2:12 into the game after Mark Recchis pass from behind the net got past Vokoun.
Kovalchuk doubled the Thrashers lead by sending a trickler through Vokouns legs.
Florida made it 2-1 at 18:32 of the first. Lehtonen made a pad save on McLeans wrist shot, but the rebound bounced in off the skate of Thrashers defenseman Ken Klee.
McLean scored again at 8:40 of the second when his flip from the back of the net went in off the back of Lehtonens leg