The Fireweed got introduced to the World of Warcraft for the expansion, Warlord of Draenor and the character requires a skill level of about 550 in herbalism to maximise their effort when farming Fireweed. It appears to be very similar to the Frostweed apart for the reddish orange colour. I can be obtained by combining ten Broken Fireweed Stems together to form one Fireweed or you can farm Fireweed at Frostfire Ridge. Judging from the name of the herb it is obvious that it is usually found near volcanic or near lava. It should be easily spotted as the ground is usually in a dark colour.
See below for maps and routes for where to farm fireweed in World of Warcraft: Warlords of Draenor