Read from the World of Warcraft client build files.
How Firebird's Tunic Compares
Who It Suits
Mail is the armor type Evoker, Hunter and Shaman wear. With Agility or Intellect, Firebird's Tunic suits Beast Mastery, Marksmanship and Survival Hunter; Elemental, Enhancement and Restoration Shaman; Devastation, Preservation and Augmentation Evoker.
Transmog Sets
Firebird's Tunic 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 Tunic Fits
Other Items Named Firebird's Tunic
2 other items in the current build are also called Firebird's Tunic. 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 86693 (86688 to 86698) were usually added in the same batch as Firebird's Tunic: the same quest line, zone, vendor or reward set.