Foam Filling is the practice of filling tires with a polyurethane foam. This is done to make them heavier and makes them immune to punctures due to no longer needing air to keep their shape. It is generally only done to agricultural tires and only the smaller front tires of agricultural machinery. The main disadvantages is that the tires need to be sawn off the wheels when changed and these hard packed tires will give an extremely harsh ride. The extra weight lowers the center of gravity of an agricultural vehicle and this can be advantageous to lower the chances of a vehicle flipping.
This is almost always done as a DIY job and has little penetration in the professional tire fitting market. It is in sometimes used an alternative to liquid ballast.