Armstrong scored his second of the season to restore Atlanta's one-goal lead just 24 seconds after Montreal drew even for the second time in the third.
Rich Peverley had a goal and two assists, including his third point of the game with a cross-crease setup pass on Pavel Kubina's power-play goal that gave the Thrashers a 4-3 edge 4:10 into the third period.
Tomas Plekanec drew Montreal even at 4 with his fourth goal at 12:40.
The Thrashers (6-4-1), who played their third game without injured captain Ilya Kovalchuk, won their second in a row after a four-game losing streak.
Kovalchuk, who leads Atlanta with nine goals in eight games, was lost for three to five weeks after he broke his foot in a 4-3 loss to San Jose on Thursday.
Carey Price stopped 25 shots for Montreal (7-8-0).
RED WINGS 2, BRUINS 0
In Detroit, Henrik Zetterberg and Tomas Holmstrom scored in the first period, then Chris Osgood finished with his first shutout of the season to give Detroit win over Boston.
Osgood made 29 saves. He also got help from the posts behind him to prevent a goal in both the first and second periods for the 50th shutout of his career.
The rebuilding Red Wings are 3-0-1 in their past four games and have a point in seven of their past eight games.
Tim Thomas had 24 saves for Boston, which fell under .500 with their third loss in four games.
LIGHTNING 2, MAPLE LEAFS 1
In Toronto, Ryan Malone scored at 2:21 of overtime, giving Tampa Bay a victory over Toronto.
Malone tapped in an awkward shot that bounced over Jonas Gustavsson moments after John Mitchell nearly won it for the Maple Leafs on a pretty rush, with the puck nearly taking another funny hop into the goal.
Vincent Lecavalier also scored for Lightning (5-4-4), who won their first road game of the season (1-4-1).