Read from the World of Warcraft client build files.
How Firebird's Helmet Compares
Who It Suits
Mail is the armor type Evoker, Hunter and Shaman wear. With Agility or Intellect, Firebird's Helmet suits Beast Mastery, Marksmanship and Survival Hunter; Elemental, Enhancement and Restoration Shaman; Devastation, Preservation and Augmentation Evoker.
Transmog Sets
Firebird's Helmet is part of the transmog set Regalia of the Firebird (Shaman only). The other 16 pieces:
Built from the World of Warcraft client build files and recorded Auction House snapshots.
Where Firebird's Helmet Fits
Other Items Named Firebird's Helmet
3 other items in the current build are also called Firebird's Helmet. Same-name entries are usually a different rank, faction, quest stage or reward track of the same object.
Blizzard numbers items as they are created, so the named items next to ID 87136 (87131 to 87141) were usually added in the same batch as Firebird's Helmet: the same quest line, zone, vendor or reward set.