More of a tip than a guide.
On my server (Shadowsong EU) Firewing Signets sell for nearly as much as Sunfury Signets do. However, Firewing Signets only drop from one area in the entire game, which is Firewing Point in Terokkar.
The mobs here are 63-65 with a nice mix of melee mobs and casters in the middle. This place can be very profitable IF it isn't already being farmed, due to the mobs low HP. If it is being farmed however, forget it, it isnt worth it.
The mobs here drop Firewing Signets, Arcane Tomes, Netherweave cloth and all the other random greens and blues that drop from other places.
Please note that your servers prices may be different, making this either a lot worse or a lot better.