Primal Fire

There's three places you can farm motes of fire from mobs. They are:

Elemental Plateau in Nagrand (requires a flying mount).
Northern Blade's Edge Mountains
Shadowmoon Valley (center of the map)

At the time you read this post, regardless of where you live or the time of day, there are three people on your server farming each of those spots. Primal Fires are by the most highly demanded Primal, and so you will find Frost Mages farming the mobs constantly for either tailored items or cold hard gold.

....................TRANSMUTES....................
...............................................................
.....Mana<->Fire <-Air...............................
.....................|.......^.................................
....................v........|.................................
.....Life <-> Earth -> Water <-> Shadow......
...............................................................

I had to get 8 Primal Fires for my turbo charged flying machine so I looked here for good places to farm it.

I can easily say that the Incancdescent Fel Sparks at the Throne of Kil'jaeden in hellfire peninsula are the best mobs to farm motes of fire at level 80. I tried Elemental plateau at first, but after doing 3 aoe pulls I had to wait for over 5 minutes for them to start respawning again ... waste of time.

However, at the throne of Kil'jaeden there will always be fel sparks to kill. It seems like they are forced to respawn if all of them are about to die. The drop rate seemed to be the same as Elemental Plateau but you have 0 downtime (apart from the little flying between each mob). Just bind your flying mount button to a easy to reach key and start killing those fel sparks. I had my flying mount bound to the middle mouse button while I was at this so I could mount easily and fast after each mob.

I got my 8 primal fires in about 20 minutes at the throne of kil'jaeden.