It's not enough to just have fire detectors. While they can dectect smoke, they cannot detect carbon monoxide, a colorless, tasteless, and odorless gas that can kill. Carbon monoxide can enter your home through malfunctioning fuel-burning appliances such as furnaces, dryers, and gas water heaters. Also, propane grills, wood-burning stoves, and fireplaces need to be watched.
Carbon monoxide detectors have been on the market since the early 90s, but accord to Better Homes & Garden article Carbon Monoxide Dangers in Your Home only 27% of American homes employ carbon monoxide detectors. These days, they are cheaper than ever.
You can find a combination carbon monoxide detector and smoke detector like this one for less than $75. If you just need a carbon monoxide detector, try the KIDDE 9C0-5 Battery Operated Carbon Monoxide Detector for under $50.