Elder Elemental - Fire

Warlocks bonded to the Elemental Powers gain certain benefits from their particular Element as well as some that are general to all Elemental Warlocks. Elemental Warlocks also gain expanded spell lists specific to their particular Element.

At One With The Elements

Having sworn their allegience to an Elder Elemental, Elemental Warlocks gain the benefits of the Endure Elements spell. Essentially they are no longer affected, and do not need to make Constitution saving throws, to stave off the effects of extreme enviromental temperatures. They are still affected by such things like smoke, lack of air, etc. This ability does not affect Fire, Cold or any other kind of damage, be it spell (Fireball) or enviromental (falling into a bonfire). 

Warlocks of Fire also can cast Faerie Fire 3/short rest at their Warlock level. They also tend to "run a little hotter" than the common folk. 

Fire Charmer

In reward for their continued servitude and devotion, a Fire Warlock's patron grants them the ability to Control Flames at will, as well as gain Advantage on saving throws realted to fire damage.

Friendly Fire

At 10th level a Fire Warlock becomes completely immune to fire damage from any source, the exception being burning damage from a Fire Elemental, summoned or otherwise. Fire Warlocks can also attempt to wrest control of a summoned fire elemental from the summoner. This is done by rolling opposing Knowledge Arcana checks, the Warlock is able to add their Charisma modifier to the roll; on a success the elemental is now under the Warlocks control as if the Warlock had cast the spell originally, however any time already past from the initial casting is still counted. This can be done once per long rest. Adversely, Fire Warlocks take half again as much damage from spells and effects with the Cold descriptor.

Soul of Fire

So close to being completely consumed by their Patron's Will and Power, Fire Warlocks that achieve 14th level and higher have been reported as bleeding flames when cut. Their granted Immunity to Fire evolves even further and now, instead of taking damage, they are healed half of whatever damage they would have taken. ie: a Fireball dealing 24 points of damage instead would heal the Warlock 12 points instead. Hitpoints healed beyond their hitpoint maximum are saved as Temporary Hitpoints until consumed by damage, replaced by a greater value, or rounds equal to half the Warlock's level. Alternatively, a Warlock can also use this ability to invigorate themselves, effectively erasing a level of exhaustion for every 10 points that could have been healed.

The cost? The Warlock now takes double damage from spells and effects with the cold descriptor.