Volatile Fire in Twilight Highlands
Twilight Highlands is the best place to farm Volatile Fire. Kill Enslaved Infernos and combine it with fishing in Pool of Fire.
If your fishing skill is only 1, you can still fish in Pool of Fire. Each Pool of Fire contains 2-4 catches, each of which will be 1 or 2 Volatile Fire. The location have 4 pool spawning points. No more than 2 pools will be visible at one time, and none may be visible if pools have recently been fished empty. Pools re-spawn approximately 12-15 minutes. You can find the Pools around Lord Canon.
The Pools are farmed by a lot of players. If you can do this alone, you will get 100+ Volatile Fire in an hour, try this at between 3-5 AM.
Volatile Fire in Mount Hyjal
Flame Ascendants in Mount Hyjal have low HP and they respawn instantly most of the time. I was often able to kill 5-6 at the same spawning point, because the mob respawned before I even looted it.
Unbound Emberfiends in Twilight Highlands is another good alternative, these are a lot harder to kill than the mobs in Mount Hyjal.