Fyrestone is in the northern section of the Arid Nexus - Badlands area. This is the place where the original Borderlands 1 started. First you'll need to head to the red building to the north west of the town (The bus stop in the first Borderlands). Once you've found that, this is where the chest lies.

Now, simply jump up the building and onto the roof to find the chest. Simply open it to get some nice loot and the achievement will unlock.