Drywall – 1/2″ drywall over firewall in kitchen

This will likely be a location specific issue, so it doesn't hurt to check your local codes.

All the fire rated work that I've done has been between units (both to other units and common areas) and any load bearing structures of a multi-unit dwelling. When we do that, fire blocking is installed in the ceiling space as well to prevent a fire from traveling through the ceiling to another unit. For everything else within a single unit, and in a single family home, we don't use any fire rated drywall.

Edit: A few reasons I'd disagree with Eric on this one. First, I'm presuming you already have a certificate of occupancy (or someone did) to move in with an unfinished utility room. That required an inspection that presumably allowed the space as is. Next, most codes for combustion devices (furnace, gas hot water) require that you have ventilation. Installing a vented door is perfectly acceptable and won't provide any fire stopping. And, finally, most HVAC's (if the furnace is forced air) will be connected to vents that run to every room in the house. From the little I saw, the furnaces and utility rooms in searches likely referred to multi-unit dwellings, and that does need to be fire stopped. You also have to use fire caulk around every hole. Our multi-unit buildings also have sprinklers throughout and spring loaded vents that snap closed in event of a fire.

That all being said, there's absolutely nothing bad about installing fire rated material around the furnace, other than the extra cost and that it's a bit more difficult to cut. If you want to do it, then by all means, do so.

Edit 2: One last note, realize that fire rated drywall increases the resistance from 30 minutes that you'd typically see with 1/2" drywall to 1 hour. Fire rated doesn't mean fire proof.

I'm not 100% clear as to whether you bought the house with the cracks and the popped out nails, or whether it has happened since you bought the place. If it has happened since you bought the place then I would be concerned and have it looked into. If it was like that when you bought it the damage could have been there for decades, there's no way to know for sure unless you can speak to the former owner.

I'm assuming you've done this or you have a slab foundation, but if you have access to the foundation like a crawlspace or a basement have a look to see if there's any evidence of settling. Try and mark off the cracks and the positions of the nails in ways you can later go back and check for movement. Use a ruler to draw a pencil line perpendicular to the cracks every inch or so, and use a grease pencil for the nails, or a permanent marker. Keep an eye on them every few weeks and see what happens.