One of the biggest differences between the various outsoles that are available on all leather fire boots, is that some are all natural rubber, some are all nitrile rubber, and some are a blend of natural and nitrile rubber.
Nitrile rubber is a synthetic rubber developed in 1941 as the first oil resistant rubber. Grades with high acrylonitrile content have better oil resistance whereas low acrylonitrile content gives better low temperature flexibility and resilience. Nitrile has moderate physical properties but good abrasion resistance. Gas permeability is low. Nitrile rubber can be modified to increase certain properties of the blend, depending on what substances are mixed to the formula. Nitrile rubber is recognized as an excellent material to protect against chemical agents (i.e. nitrile gloves), and is also an excellent material to protect against petroleum products (i.e. oil & gas). Nitrile rubber is very durable and provides good fire resistance.
Natural rubber is a rubber made of latex gathered from certain trees (mainly). While natural rubber has historically been an inexpensive solution for firefighting footwear, it does present some major disadvantages:
Many of the manufacturers use the Fire & Ice soles from Vibram. This sole is made of mostly natural rubber, with about 20% nitrile rubber mixed into certain areas of the sole. The end result is a sole that is heavier than an all nitrile rubber sole, provides LESS slip resistance, LESS heat and cold insulation, LESS chemical resistance, wears out faster, delaminates faster and tend to crumble with wear and tear.
100% nitrile soles are the best option for firefighting. If your boots do not have a 100% nitrile sole, you are not getting the best.