Alex Goligoski connected on the go-ahead goal in the second period for the Stars (5-1) and contributed three assists.
Stephane Robidas, Goligoski's defensive partner, added a power-play goal. Loui Eriksson and Vernon Fiddler also scored for Dallas.
Trevor Daley contributed a power-play goal at 14:15 of the third period while Dallas skated 5 on 3, and Tom Wandell's goal with 1:36 left completed the rout.
Rookie Jonathan Huberdeau scored his third goal of the preseason for the Panthers (1-4-1), whose regular season begins at the Islanders on Oct. 8.
Florida goaltender Jose Theodore gave up all seven goals on 37 shots.
Stars coach Glen Gulutzan went with many of his regulars, including Lehtonen and the team's top defensive pair of Robidas and Goligoski.
Lehtonen, who'll open the season as the Stars' No. 1, was sharp in his final home tuneup for the regular season opener on Oct. 7 when the Stars host Chicago.
Rookie goalie Richard Bachman played the final period for the Stars, stopping the four shots he faced.
Lehtonen had a strong first period, standing his ground and making a nice stop on Kris Versteeg's breakaway with about seven minutes left in the period.
Lehtonen also denied Jack Skille on a solo rush with 1:16 remaining to keep the first period scoreless.
Ribeiro put Dallas ahead at 1:34 of the second period, converting from the left circle off Steve Ott's cross-ice feed.
Huberdeau's power-play goal drew Florida even at 1 at 9:01, but Goligoski connected from the left point 30 seconds later to put the Stars back in front.
Robidas' goal with the man advantage made it 3-1 at 17:30 of the second, and Eriksson's wrister from the slot at 6:39 of the third gave Dallas a three-goal edge.