The Bruins will play the Philadelphia Flyers in the Eastern Conference semifinals for the second consecutive year.
Tim Thomas stopped 34 shots for the Bruins, who lost the first two games of the series at home.
Carey Price, who played junior hockey for the Tri-City Americans from 2003-07 and was the No. 5 overall pick in 2005, stopped 30 shots for Montreal.
Horton, the third overall pick in 2003 by Florida, scored with a slap shot off a pass from Milan Lucic, setting off a celebration on the Bruins bench and in the stands. It was Boston’s third overtime win in the series, including Game 5 on Saturday night when Horton scored 9:03 into the second extra period.
Johnny Boychuk and Mark Recchi scored in the first 5:33 of Game 7 to give the Bruins a 2-0 lead. But Yannick Weber scored on a power-play goal in the first and Tomas Plekanec tied it with an unassisted short-handed goal.
After Chris Kelly scored to give Boston a 3-2 lead, Montreal’s P.K. Subban tied it in the final two minutes — again on the power play.
At Lightning 1, Penguins 0: Dwayne Roloson made 36 saves for his second playoff shutout and Sean Bergenheim scored the lone goal in the second period as No. 5 Tampa Bay eliminated fourth-seeded Pittsburgh, which played since early February without injured superstars Sidney Crosby (concussion) and Evgeni Malkin (knee).
Roloson became the second goalie to go 6-0 in elimination games.
Vancouver general manager Mike Gillis was fined by the NHL for complaining about the officiating in the series win over the Chicago Blackhawks. ... Detroit center Pavel Datsyuk, Vancouver’s Ryan Kesler and Chicago’s Jonathan Toews are the finalists for the Frank Selke Trophy as the best defensive forward. Datsyuk has won it the previous three seasons.
Nashville at Vancouver (series opener), 6 p.m., CBUT, Versus