Skill Trees

The Fire Claws Werewolf can be leveled directly, although the Build becomes noticeably smoother once Fire Claws and its main synergies receive enough Points. Early on, use basic melee attacks and Feral Rage to progress through the first levels. Once charged, it lasts for 30 seconds and provides a large Movement Speed and Life Steal bonus, so you only need to refresh it periodically before returning to attacking.

Your main priority should be Fire Claws, followed by Molten Boulder and Armageddon, which are its two current Damage synergies. Lycanthropy should be developed alongside them for the substantial Life bonus, while Werewolf can be increased later to improve Attack Rating, Damage, and overall shapeshift performance. Armageddon is especially useful because it also adds continuous AoE Damage around you while you attack.

After Fire Claws, Molten Boulder, Armageddon, and Lycanthropy are completed, invest the remaining Points into Werewolf until it is eventually maxed. Feral Rage can remain at one Point, since additional levels are not required for its main purpose: maintaining the 30-second Movement Speed and Life Steal buff. Refresh it when necessary, then spend the vast majority of combat using Fire Claws.

Gear

Hellslayer is the main endgame Weapon for the Fire Claws Werewolf. Its high physical Damage and Enemy Fire Resistance reduction fit the Build perfectly, while IAS corruption and sockets are especially valuable for reaching the desired Werewolf attack breakpoint. Earth Shifter and Tomb Reaver are strong alternatives before you obtain an optimized Hellslayer.

Jalal's Mane is the safest Helmet choice, offering Shapeshifting Skills, Faster Hit Recovery, Attributes, and Resistances in one slot. Cerebus' Bite is more offensive, while a strong Rare or Magic Druid Pelt with Fire Claws, Shapeshifting Skills, and sockets can be a premium alternative.

For Body Armour, Arkaine's Valor provides a strong mix of Skills and survivability. Shaftstop or Leviathan are better if you want more Physical Damage Reduction, while Duriel's Shell works well during progression.

Hellmouth is one of the best Gloves because Enemy Fire Resistance reduction directly increases Fire Claws Damage. Magefist is a cheaper offensive alternative, while Soul Drainer can be used if you want more sustain and help with the physical portion of your Damage.

For the Belt, String of Ears is an excellent default thanks to Life Leech and Physical Damage Reduction. Verdungo's Hearty Cord offers more defensive value, while Thundergod's Vigor can be useful against dangerous Lightning-heavy content.

Sandstorm Trek, Waterwalk, and Aldur's Advance are all strong Boots depending on what your setup needs. High-quality Rare Boots with Faster Run/Walk, Faster Hit Recovery, and Resistances are often the best choice once your offensive gear is already complete.

The Rising Sun is one of the strongest offensive Amulets because it supports Fire Damage and reduces enemy Fire Resistance. Mara's Kaleidoscope is a more balanced option, while a Magic +3 Shapeshifting or strong Rare/Crafted Druid Amulet can provide excellent Skill and defensive bonuses.

For Rings, Raven Frost is extremely useful for Cannot Be Frozen, Attack Rating, and Dexterity. The second slot can use Bul-Kathos' Wedding Band for Skills and Leech, or a strong Rare/Crafted Ring with Attack Rating, Leech, Resistances, and Attributes.

Charms

For the Fire Claws Werewolf, the inventory should be built around increasing Fire Claws itself rather than conventional physical melee Damage. A Druid Hellfire Torch and Annihilus are the obvious foundation, followed by Shapeshifting Skill Grand Charms and Large Charms with increased Fire Skill Damage. Which of the two is better depends on your current Skill Level, so high-end players often mix both and compare the actual Damage gain on the character sheet. 

The remaining slots should cover whatever the Build still lacks: Life, Resistances, Faster Hit Recovery, or Attack Rating. Since this is a melee character, keeping your chance to hit and defensive breakpoints comfortable matters more than squeezing every last point of theoretical Fire Damage out of the inventory.

Mercenary

Act 1 Vigor Mercenary with Pus Spitter.
The combination is especially well suited to Fire Claws because Vigor stacks naturally with the Movement Speed from Feral Rage, making the Werewolf extremely fast between packs, while Pus Spitter periodically applies Lower Resist to improve your Fire Damage against tougher enemies. 

• Pus Spitter (Weapon)
• Vampire Gaze or Andariel's Visage (Helmet)
• Shaftstop, Fortitude, or Innocence (Armour)
• Lava Gout or durable Rare/Crafted Gloves (Gloves)
• String of Ears or Verdungo's Hearty Cord (Belt)
• Rare Boots with Resistances and defensive modifiers (Boots)