It's very important to seal off all the "bypass" openings in the attic. That means all the plumbing that comes through the ceiling, the kitchen stove vent hood, the furnace vent, etc.
For the furnace vent, pick up some aluminum sheeting at the home center and cut it around the furnace vent. Seal the it to the actual vent using intumescent sealer (pick it up in the caulk section). Use intumescent foam to seal all the plumbing openings and around each electrical box the punctures the ceiling.
Intumescent foam will not burn up in a fire. It actually expands when heat is applied. Code requires that type of foam and caulk.