Smoke detectors can pose huge risks to any marijuana smoker. Emergency responders may come to your house or apartment and be less than happy when they find out it was a smoker raising a false alarm. If you can try not to smoke anywhere with a fire alarm but, if it is a must there are some easy solutions to this problem. Put a plastic bag over the fire alarm and seal it with a rubber band or tape. Disassemble the fire alarm; this would be different for every alarm so be careful you do not damage anything. Keep your smoking area well ventilated. Open windows and fans running are a huge plus.
In case your alarm does go off it most likely does not contact authorities but makes a screeching sound. If you think authorities are coming run and hide your stash as quickly as possible then spray down the room and then use a pillow or book to fan out the smoke from the fire alarm. If you think the authorities are not coming but the sound is annoying as hell and makes you really paranoid, just fan next to the detector.
Placing a bag over the detector is the easiest option. I wrote this because the smoke detector went off during 4/20 and everyone was freaking out. Nothing happened and too much Febreze was sprayed as a result. So be safe and keep calm.