Briere, signed by the Flyers to a multiyear deal in the offseason, redirected a perfect pass from Scott Hartnell in front of the Senators net for his 12th goal of the season.
Gagne was Briere's linemate at the beginning of the season, but has missed the past eight games, and 12 of the past 14, with a concussion. Briere hasn't missed a beat, posting six goals and six assists during Gagne's eight-game absence.
"For myself, it's been kind of a roller coaster this season, especially since Simon Gagne went down, it's been tough," Briere said. "I've been switching lines left and right, but the last few games it seems to be clicking. That winning goal was a perfect example. It was probably my easiest goal of the year."
Ray Emery took over in goal in the second period in relief of Martin Gerber, who allowed three goals on 25 shots. Emery surrendered the final goal and made 16 saves as the Senators lost a third straight game for the first time since last March.
"I'm not overly concerned," Senators coach John Paddock said. "You don't go from all those wins and the best winning percentage to all of a sudden being bad."
The middle loss in the current skid for the Senators was 6-5 in a shootout to the Pittsburgh Penguins last Thursday. The Senators haven't lost back-to-back games in regulation since last December.
"We know we can play better, but we'll come back Monday and regroup, work hard and refocus," Senators forward Shean Donovan said. "John is going to put us through our paces. You're never too happy [losing], but we know we've got the team in here to win lots of games so we just have to get back to basics and play good defense."
Trailing 4-3 entering the final period, the Senators did everything but tie the game as they pressed for the full 20 minutes. They had three power plays in the period and outshot the Flyers 10-7, but were unable to beat Niittymaki. Emery went to the bench for an extra attacker with a minute and a half to play in the third period, but the Senators couldn't pull even.
The Flyers' only pressure of the period came during one stretch late when they registered four shots in a 30 second span. Philadelphia held a 41-29 overall edge in shots.
"They're a lot different than last year," Senators forward Chris Kelly said. "They skate up and down hard, get the puck to the net and they capitalized on their chances more than we did. We definitely didn't get as many pucks to the net as we should have."
The Senators came out of the first period with a 2-1 lead as Redden and Heatley sandwiched goals around Umberger's score.
Redden took a point shot that found its way through several bodies and past Niittymaki at 6:33 on the power play. After Umberger tied it less than two minutes later, Heatley wound up at the top of the faceoff circle and launched a speedy shot over Niittymaki.
Hartnell tied it at 2 just 58 seconds into the second period, but Schubert gave the Senators a 3-2 lead three minutes later when he banked a shot from the corner off Niittymaki's pad and in.
"Anytime you see a goal like that, you want to get the puck to the net as much as possible," Kelly said. "I don't think we got there as much as we should have."
The rest of the period belonged to the Flyers as they outshot the Senators 19-10 and got goals from Richards and Briere to take the lead.
"We've shown a lot of character and the big thing for us is finding that consistency and playing hard every single game," Briere said. "Coming here, it's always a tough place to play and we knew they were going through a tough stretch and we wanted to jump on them."
The Senators are 15-1 when scoring first. ... Ottawa's Dany Heatley has appeared in 217 consecutive games dating to Jan. 28, 2004, while a member of the Atlanta Thrashers.
Copyright by STATS LLC and The Associated Press
Arena: Canadian Tire Centre Location: Ottawa, Ontario
Referees: Greg Kimmerly, Dan Marouelli Linesmen: Anthony Sericolo, Tim Nowak
Attendance: 20,128 (108.8% full)
Team Stat Comparison
Power Play Goals
Power Play Opportunities
Shots on Goal
Wade Redden (3)
(Power Play) Assists: Jason Spezza, Dany Heatley
R.J. Umberger (3)
Assists: Daniel Briere, Randy Jones
Dany Heatley (12)
Assists: Dean McAmmond, Chris Phillips
Scott Hartnell (3)
Assist: Daniel Briere
Christoph Schubert (3)
Assists: Brian McGrattan, Wade Redden
Mike Richards (13)
Assists: R.J. Umberger, Braydon Coburn
Daniel Briere (12)
Assists: Scott Hartnell, Braydon Coburn