Shean Donovan made it 2-0 in the second period when he skated the length of the ice and beat Dafoe.
Boston coach Mike Keenan pulled Dafoe, but the Bruins' defense was also at fault. The Thrashers continually had odd-man rushes against Boston.
Ferraro scored a power-play goal with 1:54 left in the second period to give Atlanta a 3-0 lead.
Alexsson scored 14 seconds later and the Bruins faced a 3-1 deficit after two periods.
Svartvadet scored twice in the third period, his eighth and ninth goals of the season, as the Thrashers put the game away.
The Thrashers acquired right wing Ladislav Kohn on Friday from the Anaheim Mighty Ducks for defenseman Sergei Vyshedkevich and minor league goalie Scott Langkow. ... Boston center Joe Thornton did not play. He was completing a two-game suspension for cross-checking against Montreal before the All-Star break.