Jacob Peterson cut it to 2-1 when he batted the puck out of the air at 16:24 but Toffoli scored a shorthanded goal 1:03 into the third period to give Montreal a 3-1 lead.
"It's a 2-1 game when we give up a shorty," Dallas coach Rick Bowness said. "That's shooting yourself in the foot, it's as simple as that. The power play has to be a whole lot better than that ... like, a lot better."
Dvorak extended the lead to 4-1 off a backhand at 2:38.
"I thought we were getting a lot of chances, a lot of good looks, but then we'd give up an easy one," Stars defenseman Ryan Suter said. "They were capitalizing on their chances, and we weren't."
Seguin tipped in a Joel Hanley shot to cut it to 4-2 at 3:03 but Josh Anderson deflected the puck across the line before the goal dislodged to make it 5-2 at 4:17.
Pavelski scored a power-play goal from the top of the crease with Holtby pulled for an extra attacker at 18:41 for the 5-3 final.
"Hopefully this is just a blessing in disguise in a way that we have no choice but to get our act straight on the road coming up on a long [four-game] trip here," Seguin said. "There's no excuses. We're getting down to, not do-or-die, but the stakes are pretty high."
NOTES:Montembeault's previous high was 43 saves in a 5-2 loss at the Panthers on Jan. 1. … The win was Montreal's second since Nov. 27 (3-2 against the Philadelphia Flyers on Dec. 16). … Suzuki had two assists. … Canadiens forwards Joel Armia and Cole Caufield were placed into protocol Tuesday. ... Stars forward Denis Gurianov had an assist in 16:52 after missing three games in protocol. … Dallas defenseman Esa Lindell played his 400th NHL game. … Pavelski has scored 17 points (seven goals, 10 assists) in his past 10 games. … Montreal outshot Dallas 51-22. … There were 59 penalty minutes, 39 by the Canadiens.