Volatile Fire

brb pool is on fire.

There are "Pools of Fire" at Cannon's Inferno in Twilight Highlands and in the large pool of lava immediately in front of Sulfuron Spire in Mount Hyjal. Fishing from them rewards 1-2 Volatile Fire per cast, and there are four separate potential spawn points that each pool can spawn in either location. You'll only ever find two pools at a time. However, when you "clear" one of those two pools, another pool may spawn nearby shortly after. If you aren't sharing these with other players, you'll likely witness all four pools spawn in that location.

After all four pools are depleted in one location, they'll not begin to respawn for a good 15 minutes. Keep in mind that the pools in Twilight Highlands are unrelated to the pools in Mount Hyjal, so you can fish in one zone and switch to the next. After you finish fishing in one, the pools should have started respawning in the other. (Thank you, Cryptorchild)

I personally found farming [npc=52219] along Ashen Lake (West Mount Hyjal) to be a solid place to get these. I managed the 60 volatiles I needed in about 20 min.

The drop rate is ~13% which, according to Wowhead, is the among the highest for non-dungeon mobs and I was easily able to pull groups of 3-4 at a time. The respawn rate is high enough that I never came close to running out of mobs, and so the spot could probably handle a couple other people farming alongside you without keeping you waiting.

I do believe there is a zoning requirement; I had reached the point where the Flamewake had become the Regrowth, and the elementals now spend their time bullying [npc=52177].

As with any kind of mob-farming, I would recommend using Potion of Treasure Finding for the extra drops.