If the world we live in is habitable, and hell is fire, then heaven should be freezing cold right?
Depends on your religion. In some religions Hades (hell from some other religions), is freezing cold.
It seems religions that arise in desserts have a hot kind of hell, and the religions that originate in cold places have a freezing kind of hell (Hades). Teh religion of the ancient Greeks and Romans originated from northern climates, so they had a Hades instead of a Hell.
in either case, all religiohns tend to see heaven as having a perfectly comfortable temperature, that is if they have both an after life for those who were good and bad. The Greeks and Romans had everyone goiing to Hades regardless of how they behaved in life.
If you are agnostic, you should know better that there is no hell or heaven