Tomas Nosek, Craig Smith and David Pastrnak also scored for the Bruins, who hadn’t played since Sunday’s 5-2 victory over Montreal.
Derick Brassard scored both goals for the Flyers. Philadelphia lost consecutive games for the first time this season, having won the previous six times following a defeat.
Linus Ullmark made 29 saves for the Bruins, who tied their season high with 44 shots.
The teams traded goals, hard hits and punches in a highly entertaining second period.
Forbort scored his first of the game 30 seconds into the period when his slap shot from long range deflected off the stick of Flyers defenseman Justin Braun and over goalie Martin Jones’ right shoulder to make it 2-0.
“The first one was pretty lucky,” Forbort said.
Brassard scored 52 seconds later from in front after getting set up from behind the net by Cam Atkinson to pull the Flyers to 2-1.
Then, things started to get a little chippy.
“It seems like there’s always some emotion against Philadelphia,” Boston coach Bruce Cassidy said. “I think that goes back to the ’70s. Two teams that like to play that way. Obviously the game has changed, but you still have a bit of that element.”
Philadelphia’s Rasmus Ristolainen laid a big hit on Taylor Hall near the penalty box at center ice, and Boston responded aggressively. Marchand was penalized for unsportsmanlike conduct soon after in a scrum behind the net with Ristolainen, and Philadelphia took advantage.
Brassard finished a tic-tac-toe passing sequence with Ivan Provorov and Claude Giroux and tied it at 2 with a power-play goal with 10:35 left in the period.
The intensity continued, culminating with Boston’s Charlie McAvoy winning a lopsided fight with Joel Farabee in which the Boston defenseman landed several hard right hands.
“I was just a little heated,” McAvoy said. “In the thick of it, it was something that happened. Hopefully, it lifted our team.”
Said Cassidy: “I’m sure that got us going. I know it does.”
Forbort finished the scoring in the period to give him the first two-goal game of his career, firing a wrist shot from the slot past Jones’ glove side with 3:33 left to put the Bruins ahead 3-2.
It was the fourth of the season and just the 12th in 345 career games for Forbort, who deflected any praise.
“It was just a good all-around play by the fellas,” he said.
Smith scored his first of the season 1:28 into the third to give Boston a two-goal cushion, and Pastrnak made it 5-2 with 7:13 to play.
“They have world-class players,” Brassard said. “They’ve been doing it for so many years.”
Nosek finally converted for the Bruins in a dominant first period, scoring on their 20th shot to give them a 1-0 lead with 1:35 left in the first. Anton Blidh passed from behind the net to Nosek, who settled the puck and fired high over Jones with a backhand.
Jones had been stellar to that point, particularly during consecutive Boston power plays midway through the period that included 12 seconds of 5-on-3 time. The Flyers’ backup goalie made seven saves during the successful penalty kill.
Philadelphia’s sagging power play had been 3 for 38 over 12 games before Brassard’s second goal.
Brad Marchand had an assist on Forbort’s first goal, upping his team-leading total to 20 and giving him points in eight straight games.
Bruins: Smith returned to the lineup after missing two games with an undisclosed injury.
Flyers: C Kevin Hayes (lower body) missed his second straight game after coming back from a 12-game absence and scoring in Tuesday’s 2-1 overtime win over Calgary. Hayes appeared to get injured late in the second period in that game but returned for the third. Flyers coach Alain Vigneault didn’t have an update after the game. … F Patrick Brown (dislocated thumb) missed his second consecutive game. He is contemplating surgery or rehab.
Bruins: Host the Calgary Flames on Sunday.
Flyers: At the Tampa Bay Lightning on Tuesday.