Ottawa snapped a three-game winning streak and fell to 8-1-1 since a Feb. 7 loss at Buffalo. Dany Heatley, booed throughout the game by fans still upset over his trade request from the Thrashers in August 2005, kept up his torrid pace with the puck, scoring twice on the power play for his 39th and 40th goals of the season.
His second goal gave the Senators a 2-1 lead early in the second period.
The Thrashers began the night with just six points in nine home games since a Jan. 16 victory over Los Angeles. But Belanger, who made it 1-0 in the first, forced a 2-2 tie early in the third with his 13th goal.
Pascal Dupuis' 12th goal, his first since the New York Rangers traded him to Atlanta on Tuesday, came on a breakaway backhander that ended the scoring with 4:56 remaining.
Six minutes earlier, Kozlov's 21st goal made it 3-2 with a shot from the right circle. Taking control of the puck after Andrej Meszaros slammed Marian Hossa into the rear boards, Kozlov fired into a crowded crease that appeared to block goaltender Ray Emery's vision.