edit: This information is not valid anymore. since 2008 Midsummer to be exact

Dire Maul fire is after Guard Slip'kik.
Stratholme fire is on the Scarlet side, by Timmy
Blackrock Spire fire is on the Lower part, near the Ogre Tribute pile.
Scholomance fire is in the Viewing Room.

The one in Dire Maul, is after Guard Slip'kik in Dire Maul North
The one in Stratholme is right before the entrance to the Scarlet side, its basicly at Timmys spawn.
The one in Blackrock Spire is in LBRS, near the Ogre Tribute pile..
The one in Scholomance is in the Viewing Room as InsaneRyu said..

In the end this will reward you with 10 items which will grant you a buff, - can only be used when you are near a festival fire, and they have a duration of 14 days(the one i had at least)

and a pet you can summon, which is very unique and only achievable this time at year - its an elven spirit(like the workers in warcraft 3) which is red

this quest can be taken at lvl 50

A L70 rogue and I (L67 priest) did the Scholo part in about 5 min. First we killed the first 4 skeletons on the bridge when you first zone in to get a little room to work with. Then the rogue jumped down (while stealthed) in front of the Viewing Room door. He did not aggro any mobs, and I was able to stay up on the bridge (where the banister was broken), completely safe, and heal him while he cleared the group of mobs in front of the door. Once that pack of mobs was dead, he picked the locked door, I jumped down, and we got the flame.

The rogue had maxed lockpicking, and probably could have soloed it, but it's always nice to have a priest along, esp with shackle undead and abolish disease. :)

This quest was a part of the Midsummer Event until 2007; it is no longer available.