Oracle Text

Exile all permanents. Exile all cards from all hands and graveyards. Each player's life total becomes 1.

Card Rulings

7/1/2012 These actions are performed in order, but no triggered abilities resolve in between them and no player may cast spells or activate abilities in between them. This means that if a permanent’s ability triggers on a player losing life, that permanent will be exiled first and won’t be on the battlefield when your life total becomes 1. Leaves-the-battlefield triggers won’t resolve until you’re done resolving Worldfire entirely.
7/1/2012 If your life total is greater than 1 when Worldfire resolves, the change in life total will count as life loss. Other effects that interact with life loss will interact with this effect accordingly.
7/1/2012 Worldfire won’t be exiled. It will go to your graveyard as the final part of its resolution.