MONTREAL -- The Montreal Canadiens keep finding ways to come back after falling behind.
Parenteau scored his second goal of the game into an empty net with 19.4 seconds remaining.
"It's nice to get everybody on the scoresheet," Pacioretty said. "We have a lot of depth on this team, and you want everybody feeling good about themselves offensively, and that's definitely the case tonight."
Sekac scored his first NHL goal at 18:11 of the second to tie the game at 3-3. Parenteau scored his first goal in five games with the Canadiens with 23.2 seconds left in the period to give Montreal its second lead of the period.
"Well, those two goals were huge," said Gallagher, who had his first multigoal game in his 130th regular-season game. "It was obviously the difference between being down by one or up by one going into the third period. When Jiri scored, I think everyone gets excited. It was his first NHL goal, and we can share in that excitement, and then PA, same thing, first goal as a Hab. So I think two exciting moments for those guys, and as teammates it was fun to share with them."
Carey Price made 25 saves for the Canadiens, who improved to 4-1-0. Montreal defeated Boston in a seven-game Eastern Conference Second Round series in last season’s Stanley Cup Playoffs.
Parenteau's second goal was the Canadiens' second in three power-play opportunities in the game.
It came after Boston's Milan Lucic was called for boarding at 18:40 for his hit on Emelin. Lucic, who gave the Montreal defenseman an earful in the handshake line at the end of Game 7, was given a misconduct penalty after Parenteau's second goal.
"There were a lot of mistakes where guys should know better," said Bruins coach Claude Julien, who declined to comment on Lucic's penalty. "They were trying to do a little too much or not focused enough. We've got to make sure we fix those things."
Tuukka Rask stopped 18 of 23 shots in his second start in as many nights after making 18 saves in the Bruins' 3-2 shootout win at the Detroit Red Wings on Wednesday. Niklas Svedberg replaced Rask after Gallagher's second goal put Montreal up 5-3 at 7:17 of the third. Svedberg finished with two saves.
"Everything was under control, then a couple of breakdowns and it's a 4-3 game," Rask said. "So it obviously kind of shakes you a little bit. But then again we wanted to come out strong in the third and shake those goals off, but the start of the third wasn't the sharpest, then we gave up a goal, and that was it."
Including the playoffs, Rask has one win and a 3.19 goals-against average and a .890 save percentage in his past four games in Montreal.
"I've got to do something about it," said Rask, who is 3-11-3 against Montreal during the regular season. "I'll figure something out."
Simon Gagne, who signed a one-year contract with Boston on Monday, drew the Bruins to within one at 14:11 of the third with his first NHL goal since April 23, 2013.
"He played well [Wednesday] and tonight he scored a goal," Julien said. "So I think we're satisfied with what he's bringing so far."
Gallagher gave Montreal its first lead at 2-1 with his second goal of the season at 7:43 of the second.
Soderberg tied it with his first goal on a shot from the slot at 8:34. Eriksson's goal restored Boston's lead at 11:31. He made it 3-2 when he deflected Torey Krug's pass from the left side past Price.
Chara scored his first goal of the season on a power play midway through the first period. The Bruins captain deflected David Krejci's one-timer from the point past Price at 9:30 on Boston's second power play. Chara, Krejci and Lucic each recorded their first point in six games on the goal.
Pacioretty scored his second goal at 11:33 of the first period to tie the game at 1-1. It was the Canadiens' first power-play goal in 15 opportunities this season.
"The power play was good," Montreal coach Michel Therrien said. "Finally we capitalized on our power play, but I liked the spirit. I liked that we were more aggressive to the net and we got some chances."
The four goals were a season-high for the Bruins, who have scored six times in two games after scoring twice during a three-game losing streak.
Referee Graham Skilliter immediately waved off an apparent goal by Montreal at 7:20 of the second. Skilliter ruled that Canadiens center Lars Eller had both interfered with Rask and pushed the puck into the net with his glove.
Center Gregory Campbell made his season debut for Boston. He missed the preseason and Boston's first five games because of a core injury.
Hall of Fame goaltender Ken Dryden brought the Canadiens' symbolic torch from the dressing room to launch the pregame ceremony. Dryden, who won the Stanley Cup six times with Montreal during the 1970s, was given a rousing ovation before passing the torch to Price.