Marner's two goals, at 30 seconds and 1:07 of the second period, gave Toronto a 3-1 lead.
"That shorthanded goal was unbelievable," Keefe said. "A big moment in the game, it was a critical time in the game. It was a big-time play and we kept going from there."
Colin Blackwell made it 4-1 at 2:04 to chase Bobrovsky before Jake Muzzin made it 5-1 at 8:40 in his first game since Feb. 21.
"You don't want wild swings in a game where we had control," Maple Leafs forward Jason Spezza said. "When we have control like that, you want to lock it down. You have to learn from it. It's a team we're likely to see again."
Reinhart began the comeback when he made it 5-2 at 11:52 before Radko Gudas scored shorthanded at 12:51 to make it 5-3.
Giroux scored at 17:56 to make it 5-4.
"My teammates were doing a good job of reminding me I didn't have a goal yet," Giroux said. "So, it's nice to be able to get a goal and help the team have a comeback like we did tonight."
NOTES: The Panthers, who became the first NHL team to clinch a playoff berth Sunday, set a team record with 104 points. They had 103 in 2015-16. … Florida forward Patric Hornqvist had one shot on goal in 5:51 of ice time after missing six games because of an undisclosed injury. … Matthews has 17 goals and 10 assists during a 14-game point streak. … Marner extended his multipoint streak to seven games, tying a Toronto record shared by Matthews (2018-19), Doug Gilmour (1992-93), Vincent Damphousse (1990-91), Rick Vaive (1983-84) and Corb Denneny (1919-20). Marner has four goals and 15 assists during the streak. … Keefe said Kallgren was healthy enough to return if needed.