Firemaw is probably the first encounter where healers can really benefit from having ui-mods like bigwigs installed. As most of people have pointed it out, having more than one offtank usually just complicates OT healing. Also letting OT to run out to let flamebuffet wear out is probably a good idea.

On a side note, I once changed characters with a lvl60 epic druid for one BWL, despite it being my first healing experience ever, I managed to top the effective healing done on MT with just "rejuve, Spam HT4, Swiftmend on Shadowflame". Swiftmend is the key here, a clever resto druid with bigwigs / similar and quick reflexes pretty much trivializes the encounter...

As previously mentioned, you should have only 2 OTs. Assign one dedicated healer to each of those, directly responsable for keeping them alive, preferably priests for PW:S emergencies. If they have the mana to spare, toss some renews on the rogue/DPS warriors if they cant bandage, but the main priority should be their assigned OT. If you tank Firemaw in the usual position - lower left corner of his room, the OT healers should be standing in his room too, but to the right of the door, such that they are out of LoS from Firemaw yet fully in LoS of the OTs (who themselves are positioned on the right side of Firemaw).

Have the rest of the healers on the MT. Assign one priest to do a PW:S when the Shadowflame warning comes up, and another priest do a GHeal so that it lands exactly after the Shadowflame. A druid using Swiftmend after the Shadowflame is also good. Ofcourse, you need to make sure no other priest uses PW:S so there is no debuff at that critical moment, as well as no other druid consuming that Rejuv. The rest of your healers should focus on doing topping off - chain cast low mana low heal heals, HT R4 or Heal R2, that should be continuously hitting the MT throughout the fight.

as a few people have already said ... I think your main problem is the offtanks

we use the same tactic as a few other people here in that we have 2 offtanks. They hide in the little corner near the alchemy lab most of the time (with a priest and a druid also there as OT healers) and run in about 15 seconds before a wing buffet, do their taunt, and then run back out again as soon as the boss goes back on the the MT

having them sitting taking damage there all the time seems like a waste of healing. If they run in and out when needed then the OT healers only have to worry about healing them for about 15 seconds at a time. When they see the OT start to move in they know to get ready with the healing. When he moves out they just heal him back to full and wait till the next turn.

If your offtanks don't have maxxed FR (315 buffed), just rotate them in and out of line of sight - you don't need the same offtank every time. Just make sure your healers know what they're doing.
Doing this lets the OTs keep their debuff below a safe number - you can determine what that is - say 5 or 6.

If you don't swap your MT (and it's not needed) sometimes his debuff stack will get ridiculously high, just because your resists get timed poorly (25 debuff stacks is certainly not out of the realm of possibility). If that starts happening, the single most effective trick I've found is teach your MT how to clear his stack with a GFPP pot. If you have around 10-12 stacks, wait till the stack time is around 10-15 sec, pop the pot and you'll absorb all the fire damage causing the stack to not refresh - and therefore reset.

The only place you're going wrong is that you have too many OT's and you have your OTs out all the time. That means they take huge amounts of damage for no reason, which means you have to stack healing on them, which means there isn't enough on the MT.

You only need 2 OTs (can get away with one even), and they should hide behind the pillar at Firemaw's left leg most of the time so they don't get the debuff.

Their ONLY function is to run in at the right moment, taunt, eat the wing buffet and then go back into the safe spot. Frankly, they don't even need a weapon (though it's useful to have one for Mocking Blow if Taunt is resisted).

Get that right, and the rest follows. You'll have fewer OTs, each taking almost any damage, you'll have more healing on the MT, and you'll be able to get the rest of your warriors doing proper DPS and running in/out with your rogues.

As an alternate, the other off-tanking tactic is to have the MT (with his back to the gate) take all the wing buffets, then taunt Firemaw back from the off-tank. That makes it VERY predictable when the off-tank is going to take damage and it means no-one gets knocked back anywhere, but the downside is that threat is limited by what the off-tank can generate (and this will be even lower if he needs to run out to drop flame buffets).

I've actually never been aware of this. Does the same trick apply to Huhuran's tank dot and other elemental based stacking dots?

Yes, it's a standard game mechanic I believe. Any time you absorb *all* the damage from an effect, any secondary effect that might have taken place is removed too. In this case, if you absorb all the damage from his aura, it doesn't trigger the debuff.
If you fully block a talented revenge, for example, the stun proc won't go off. But if one damage gets through, it can proc.

For huhuran it's not really practical though because she puts out a bunch of auxiliary nature damage - meaning it will probably eat the absorb pot much faster - making it harder to actually time it properly.

It's actually quite relevant for alliance that sometimes uses BoP to force a temporary MT switch on huhuran, but the BoP has to be timed to the millisecond or requires you to resist once to clear the stack. If a NR pot can make one resist automatic, it would give you a lot more leeway on BoP timing.

Nowadays KTM makes it far easier to balance your tank threat and pull off timely non-taunt transitions, but it was one of the two things that held us back when we were initially learning her.

Erm, unless something has changed on Firemaw, absorbing all the damage does *not* stop the secondary effect, of applying the debuff, from working. I know this because I use Fire Ward all the time, and even though it fully absorbs all the debuff damage, you still get hit by the debuff. I've heard this rumour a lot, but whenever I've tested it using either pot or spell, it hasn't worked.

The only place I know where this is definitely true is Ragnaros - if you absorb all the damage from one of the explosions from his fireballs, you don't get thrown around the room.

Even though you reduce a lot of stress at the healingcamp by letting the OT's only get into LOS for WB-taunting, we will lose the failsafe option of a tankswitch when the MT goes down or eats unlucky/shitty taunted buffet(s). One little mistake would result in a certain wipe since half the raid (read: the whole raid) is too far above the 2 OT's threatwise.

LOS rotating debuffs after eating a wing buffet seems a bit more safe, even though it requires a bit more effort from the healers, amirite?