Like many other items, Firewalker Boots has been implemented a long time ago (12 years) and lost a lot of its real usefulness, a time that Tibia was different, practically another game.
The utility is only to defend against fire field damage, at that time could make a big difference between waste and profit in a DL hunt for example. Unlike today, that the waste made there can be easily reversed, or you can just not hunting there and hunting in better places (which there were not so many in the past).
The only practical place I see a good use for it are hunts from DL, that were great hunting grounds at the time, and are still good.