Fun fact: if you leave food in stove and fire starts AFTER you told to a sim to call emergency line, but BEFORE the sim actually make the call, the firefighter will run to extinguish the fire, then still fine you for calling when there was no emergency! EPIC!
Double check the stove BEFORE leaving home
A sim called taxi while another sim was cooking, leaving the food on stove which burned. Firetruck arrived right when all family left home. After several days spent in community lots, family came home with taxi and found the firefighter right when he was going to extinguish the fire (this is normal in The Sims 2, the lot is saved when sims go to community lot, this cannot be done in The Sims 3).
Done on purpose, had to try several times before succeeding.