Discuss your purpose
Why are you running the Firelands trash runs? What do players need to know about this scraggy band of Horde or Alliance, venturing into Ragnaros' domain? Well, you're there for two purposes: Avengers of Hyjal reputation, and BOE epics and profession patterns. You can get up to honored reputation with the Avengers from most trash and further beyond that on Molten Lords and Ancient Core Hounds. At friendly, you'll be able to get some sweet ilevel 378 cloaks, and at honored you're graced with a handful of belts to choose from.
Let people know up front what the raid is all about, or ask early how long you're going to be farming, etc.
Need before greed?
Set roles early, and let people know the rules ahead of the game. If epics drop, you're going to have hungry raiders scrambling for upgrades. Set the rules early, and then communicate with your groupmates about who gets what if it drops. If tanking epics drop, don't let "main" spec DPSers fool you into thinking they need the item -- you'll probably just see it on the auction house later.
Patterns and professions
Epic crafting patterns that use the new Firelands crafting materials are dropping from Firelands trash mobs. Let the trash raid leader know early what profession you are, and make a note of who has what profession so that you can dish out the patterns to people who can use them. Otherwise, again, you'll probably see it up on the auction house.
Making money isn't a bad thing
If no one needs an item, your best bet is to greed roll for it. After all, everyone in the group is putting in the effort and time to get these epics and patterns, so you'll want to reward players with a shot at these potentially lucrative items. The auction house will be buzzing with purples for a few weeks while the Firelands are pillaged and pilfered of its BOEs.
A learning experience
Above all, trash runs are a great way to learn the trash and the layout of an instance before actually stepping in with your guild. Here's how to deal with the common trash packs at the beginning of the instance:
Molten Lord This molten giant will apply a debuff stack to your tank that deals significant damage over time, so have a second tank taunt off at around four to five stacks and trade the Molten Lord back and forth when stacks drop. Ranged will have to watch out for his magma jets, and melee have to move for his stomp.
Ancient Core Hound These dogs will be familiar to those who had the pleasure of running Molten Core back in the day. Core Hounds have a fear, a nasty flame breath, and an ability called Dinner Time that will devour your tank and spit him out. Pour on lots of healing, and try to mitigate the fear with Tremor Totems, Fear Ward, or other fear-breaking abilities.
Molten Surger Surgers charge random players and do AOE damage. Spread out to avoid taking too much damage to your whole raid.
Hell Hounds These guys are tricky. Suppress the urge to AOE them down; instead, have your tanks pick up a few each and focus-fire them down. They hit harder over time as a group, so getting them down one at a time turns out to be more effective then letting them live as a group longer.
Flamewalker packs The Flamewalker packs are comprised of Flamewaker Cauterizers (healers) that need to be shut down before the other creatures. Interrupt and stun them. You can even CC one of them before the pull. The rest of the mobs, the Forward Guard and the other guy, are easy to tank in a group with AOE tanking abilities, and your healers shouldn't have too much trouble keeping your tanks up. They look more intimidating than they are.
Unbound Pyrelords and Unbound Smoldering Elementals Arguably the trickiest trash pack, these elementals have a unique gimmick. The baby unbound smolderings will take 90% less damage unless ignited by the big guy. The Pyrelord will Ignite one of his minions at 80%/60%/40% health. You'll want to stop using stuns on the Pyrelord because any missed ignites will force you to take a long time to DPS down the babies. Bring the big guy down to 80% health, then kill his Ignite target. Repeat for 60% and 40% health. Also, the ignited elementals AOE massive fire damage, so kill them quick.
Turtle packs Ignore unless your PUG wants to bounce around like crazy people and fly off of the Firelands. For the love of all that is holy, run. If you must kill the turtles, have everyone stand on the edge, aggro the baby and pray your group has a sense of humor. The patriarch turtle has a spell reflect shield that shoots back spells at the caster. The matriarch and hatchlings have a shell spin and knockback that is very annoying. Use lots of stuns and you'll be fine against this shelled menace.
Giant Fire Scorpion and his friends Tank and spank the scorpions but watch out -- the little guys explode when you kill them. Get out of range, and kill them one-by-one.
There are other trash packs, of course, but these are the ones you are most likely going to encounter. Good luck, and have fun pillaging Ragnaros' domain for all that fun loot.
The news is already rolling out for the upcoming WoW Patch 4.2! Preview the new Firelands raid, marvel at the new legendary staff, and get the inside scoop on new quest hubs -- plus new Tier 12 armor!