Antoine Roussel scored twice as Dallas won in San Antonio, home of the Panthers' American Hockey League affiliate.
Roussel nearly had a hat trick, but officials ruled his third-period shot was deflected into the net by Colton Sceviour.
"I thought the energy was really good today," Roussel said. "I think everyone competed hard. We put pucks at the net, we dug in and we had a lot of success doing that."
"I thought we played well. I thought we competed well," said Stars coach Lindy Ruff, whose team improved to 2-0-1 in the preseason. "We were a little sloppy in the first period, didn't play well in our D zone, but after that I thought we played a great compete game. I thought we did a lot of good things. … I thought our young D was the key. I thought they had a strong night for us."
Defenseman Tom Gilbert scored Florida's goal, beating Dan Ellis midway through the third period to make it 3-1, but Mueller's goal with 5:50 remaining capped the scoring.
"We've been working on some system things and I saw a couple of snapshots of that tonight of some good things and some things that we need to continue to work on as a group," Florida coach Kevin Dineen said. "We looked a little disjointed as well. We're still in that process of evaluating. The turn of finding the chemistry on the team is going to have to happen pretty quick."