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.
That's a really tough thing to play through; when you finally are thrown out there, the game is fast, intense, every single play matters, you've got to find a way to play through that and make smart plays. So it's a huge testament to [Vermette] and some other individuals in our locker room that are putting their own ego, their own wants and needs aside, and are ready for those opportunities when they do come.
— Blackhawks captain Jonathan Toews on teammate Antoine Vermette, who returned from being a healthy scratch in Game 3 to score the double-overtime winner in Game 4