Came home today and our First Alert model 9120B smoke detector/fire alarm was chirping 5 times every 45 seconds. Changed the battery like Youtube said, but to no avail.
Turns out it was the carbon monoxide detector located near the smoke detector that had gone bad. 9120B smoke detectors chirp 3 times when their having issues, not 5 times. Thanks to the First Alert hotline for figuring this out (1-800-323-9005 option 1).