What Firebloom is used for
Firebloom is used by Alchemists to make high level Potions and Oils, like the “Elixir of Superior Firepower” and “Brilliant Wizard Oil” and this is why Firebloom farming is great. It can be gathered by herbalsists from skill 205. Firebloom generally grows quite spread out and in open areas of the zones. On my Server, a single Firebloom is worth 20-50 Silver, so a stack of it would be 4-10 Gold.
Tanaris Firebloom farming
While the zone is quite large, Tanaris is by far the best zone for Firebloom farming. Having a fast mount makes a massive difference here, since the travel times between the nodes is so large. In addition to the Firebloom, if you have Mining, too, you can get quite a bit of Mithril Ore and even some Thorium here.
Farming Firebloom can also be done in the Searing Gorge, though the Cauldron might be a hinderance here. You can either make whole circuits around the zone or leave the cauldron out. Since the spawn locations are quite evenly spead out, I recommend including the cauldron. Since the Searing Gorge is not that large, you might have some extra time to kill the Fire and Magma Elementals on the route. These can drop Elemental Fire/Earth, which can sell for some extra gold.
If you herbalism skill is already at max level, or at least at around 250, it can be worth it to just do whole circuits of the Burning Steppes. This is because, apart from farming Firebloom, you also get Sungrass and Gromsblood here. These two herbs can also be sold for some extra gold, though since the zone is quite small, there are fewer spawn locations. If you go farming here, you might be waiting for respawns sometimes.