AN ACR LGT HAD A FALSE FIRE WARNING IN THE AFT LAVATORY.

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ON DSCNT INTO MBJ; SO RECEIVED INTERPHONE CALL FROM FLT ATTENDANT THAT 'THERE IS A FIRE IN THE AFT LAVATORY!' WE WERE PASSING 12000 FT. I INCREASED OUR RATE OF DSCNT; AND THE FO DECLARED AN EMER AND REQUESTED CLRNC DIRECT TO THE ARPT FROM MBJ APCH/TWR. THE SO SUGGESTED; AND I CONCURRED; THAT HE DON THE PORTABLE BREATHING EQUIP AND GO BACK AND FIGHT THE FIRE. HE DID SAME AND ENTERED THE CABIN; ONLY TO RETURN 15 SECONDS LATER ADVISING THAT IT WAS A FALSE ALARM. THE FO CANCELLED THE EMER. IT SEEMS THAT THE FLT ATTENDANT; IN SPRAYING THE CABIN; ALSO SPRAYED THE AFT BIFFY AND CLOSED THAT DOOR. SHORTLY THEREAFTER; THAT SMOKE ALARM ACTIVATED. A FLT ATTENDANT RESPONDED; OPENED THE DOOR; SAW AND SMELLED SMOKE; TOOK THE HALON BOTTLE AND SPRAYED THE LAVATORY; WHILE ANOTHER FLT ATTENDANT CALLED THE FLT DECK; DECLARING THE FIRE. WITHIN SECONDS; THE FLT ATTENDANT REALIZED THERE WAS NO FIRE AND TRIED TO RECONTACT THE FLT DECK BY USING THE CALL BUTTON. THE COCKPIT CREW HEARD THE CALL BELL BEING RUNG; BUT DUE TO THE URGENCY OF THE SIT AND EMER ACTIVITY BY ALL 3 COCKPIT MEMBERS; NO ONE TOOK TIME TO ANSWER THE FLT ATTENDANT'S CALL. THE SO WAS ADVISED OF THE CURRENT SIT BY THE FLT ATTENDANT AFTER HE ENTERED THE CABIN.