Gregory Campbell added a third-period goal to help the Panthers win for the fifth time in seven games. A standing-room crowd of 19,772 set an BankAtlantic Center attendance record for a hockey game.
Florida's Ville Peltonen and Montreal's Mark Streit scored first-period goals.
Cristobal Huet made 25 saves for Montreal.
Horton scored his 14th goal at 5:59 of the third to give Florida a 2-1 lead.
After Panthers defenceman Branislav Mezei kept the puck in the Montreal zone at the blue-line, Gary Roberts grabbed it and skated deep into the zone. Roberts passed to Horton, who fired a wrist shot from the left circle past Huet.
Campbell scored his second goal at 9:41, poking the puck through a crowd in front of the Canadiens' net.
Peltonen scored his fifth goal at 2:54 after taking a centring pass from Chris Gratton, who was behind and to the left of the Montreal net.
Streit knotted the score at 14:43 when his second goal came on a rebound of Chris Higgins' shot on a 2-on-1 break.
Notes: Florida C Stephen Weiss returned after missing seven games because of a shoulder injury . . . The crowd, apparently including many vacationing Canadians, seemed to favour Montreal by about a 60-40 split . . . Florida's only other sellout this season was opening night against Boston on Oct. 6 . . . The Panthers finished 7-5-2 in December.