Jakub Voracek had two assists, and Giroux also had an assist to help the Flyers snap a three-game losing streak and extend their home winning streak to six games. They were coming off a 2-3-1 trip in which they finished up 0-2-1.
Mason shut the Canadiens out for 59 minutes before Alex Galchenyuk scored with 55.1 seconds left to cut it to 2-1.
The Canadiens have lost two in a row after going 9-0-1. They followed a 6-0 loss at home to the Los Angeles Kings on Tuesday with another offensive clunker.
The Flyers didn't show any ill effects from a 13-day trip. Despite a 7-2 loss at Chicago on Wednesday, they looked fresh from the start.
Just the feel for the direction they were headed and what they're trying to do, it just felt that this is a really good thing for this organization. They've got the pieces. We can put something together and go on a run or two and be together for a while, and I'm really excited about that opportunity. The team we have here has an opportunity to win, and that's the most attractive thing.
— Washington defenseman Matt Niskanen on why he decided to sign with the Capitals