Kevin Hayes and Zack MacEwen scored for Philadelphia (6-3-2), which ended an 0-1-2 stretch. Carter Hart made 31 saves and is 6-0-2 this season.
"It's not so much that he's making the saves, it's how he looks," Flyers coach John Tortorella said of Hart. "I think a goaltender can sometimes set the tempo for your team. He looks relaxed, he looks calm. I think it settles the team down, also. He's handled himself really well."
Giroux gave the Senators a 1-0 lead at 2:44 of the first period when he tapped in a cross-ice pass from Tim Stutzle.
Giroux played his first 1,000 NHL games for the Flyers, who selected him in the first round (No. 22) of the 2006 NHL Draft, before being traded to the Florida Panthers on March 19. He signed a three-year, $19.5 million contract with Ottawa as a free agent on July 13.
"I think we do a good job of staying in games or getting leads," Giroux said. "It just looks like, at the end of games, we can't deliver. And that's going to change, but it needs to change quickly."