They are equal in strength to Dwarven boots regardless of their armor type, making them stronger than Dragonscale boots.
When a path between Termina and Hyrule was opened during the Last Great War of Hyrule, Terminan mages took the Fierce Deity's Mask with them into Hyrule, and it was separated from its associated sword and suit of armor. This was done so the pieces could be used in battle as well as to prevent the Mask from falling into the hands of the enemy.
Background and Inspiration
The Fierce Deity's Boots are retextured Dawnguard boots. They were chosen because of their similarity to the boots worn by Link while under the effects of the Fierce Deity's Mask.
The Boots were added to the mod in version 6.0 with the rest of the Fierce Deity's Equipment.