The Firelands Invasion is a world event that was introduced in Patch 4.2.0 as a prelude to the Firelands raid. The majority of the activity occurs within Mount Hyjal and the Molten Front, where players attempt to beat back forces loyal to Ragnaros that are seeking to destroy the World Tree.
Players progress through the quest line by acquiring [Marks of the World Tree]. Unlike most other forms of currency, these are used primarily for quest progression, though some items may be purchased to gain reputation with the faction city capitals, or even to buy a chance for the [Scorched Stone] to drop. These are all offered by Zen'Vorka.
It should be noted that players may not even begin the quests unless they have completed all the quests in Mount Hyjal, up to and including Aessina's Miracle. For that reason, Guardians of Hyjal: Call of the Ancients is offered to those who have not progressed as far as they need.
[85] Through the Gates of Hell and complete the daily quests listed below to earn enough [Marks of the World Tree] to continue. (150 marks at 14 per day + 15 from Through the Gates of Hell; does not count Ricket's Daily)
[85] Flight of the Storm Crows and complete the daily quests listed below (after completing four from the previous stage) to earn enough [Marks of the World Tree] to continue. (150 marks for recruiting the Shadow Wardens; if completed already, then 125 marks each for the Moonwell, Ancients, or Armaments quests)
Anren/Tholo (requires having quested for the Druids of the Talon and completing Need... Water... Badly...; quests below reflect accepting Into the Fire)
[85] The Hunt Begins and complete the daily quests listed below (after completing four from the previous stage) to earn enough [Marks of the World Tree] to continue. (150 marks for recruiting the Shadow Wardens; if completed already, then 125 marks each for the Moonwell, Ancients, or Armaments quests)