Since people still ask where they are found:
These seeds are available from the portal by Majordomo Staghelm in Firelands. They look like tiny Amberseeds anyone can see, but only Druids can loot them. Druids can loot up to five at a time, and it appears five spawn at a time.
Since Burning Seed are BoA, they can be mailed to alts, letting Druids stockpile more from future raids. (Interestingly enough, this means you can mail it to a lower-level Druid alt and they can also use it.)
You either clear to the portal (accessible after the Gate-Keeper Baleroc is dead and the bridge goes down), or simply keep a lockout extended. They also respawn after a soft reset.