Flame Cap requires a herbalist to be level 335 to gather and can only be found in Zangarmarsh in the Outlands. With a low spawn rate it makes it quite difficult to find. It seems that Flame Cap is not used in any recipes and is only consumed for buff. Consuming a Flame Cap grants the player: a chance to strike a ranged or melee target for 40 fire damage. Also increases fire spell power by 80. Last 1 min (3 Min Cooldown).
See below for areas to find Flame Caps for additional fire damage.