An Air Transat Boeing 737-800, registration F-GZHF performing flight TS-103 (dep Mar 25th) from Orlando,FL (USA) to Montreal,QC (Canada) with 102 passengers and 6 crew, was descending through FL250 towards Montreal when a rapid loss of cabin pressure occurred, the flight crew donned their oxyen masks, initiated an emergency descent to 7500 feet, the passenger oxygen masks were released, too. The aircraft continued for a safe landing in Montreal about 21 minutes later. 3 passengers received minor injuries and were taken to hospitals. The Canadian TSB reported maintenance replaced the outflow valve and the cabin altitude gauge. A test flight was conducted confirming normal function.