Fireproofing all flammable parts of buildings is necessary if a tribe wishes to avoid easy destruction. While fireproofed buildings are still susceptible to bombs, they become impervious to players who don't know how/are too lazy to make bombs. Fireproofing only works on buildings that have some flammable parts, but stone buildings usually have flammable thatching. If a fireproofed building is retooled, it will need to be fireproofed again after being placed. Objects that are not part of buildings can also be fireproofed if they are flammable. For instance, small tree stumps can be fireproofed.
How to Fireproof
In order to fireproof you first need some sort of water container, such as a bucket. Once the container is filled with water, simply click on a flammable part of a building (i.e. the thatching or the wooden walls of a hut). If everything was done correctly, one portion of water should now be removed from your container. If this did not happen, it could mean a couple of things; most likely, the part is not flammable. It could also mean that you were too far away from the item you're targeting, or it has already been fireproofed. Once everything on the building is fireproofed, it can no longer be burned down.