Old smoke detectors at the end of their lifespan that need to be replaced should not be thrown away in regular landfill trash. Many smoke detectors have small amounts of radioactive material, Americium 241, that could pose health hazards if broken open. The Solid Waste Agency of Northern Cook County (SWANCC) states on its smoke detector disposal page, "By federal regulation, all smoke detectors contain the mail-back address to send used units with radioactive chips. Call the companies prior to sending packages for fees and mailing instructions and brands accepted for disposal. Mark all packages 'For Disposal Only'." SWANCC does not accept smoke detectors in its Household Chemical Waste disposal program, but it offers links to two companies that will accept them by mail.
Here's a more comprehensive list of companies that may accept smoke detectors that they manufactured.