If I remember right ( I don't have the old books at hand ) the Firstborns had in first edition the SoK Tribal Warriors as part of the firstborn army. Then came the SoK source book...and they got their own army.
The Burning Blades have only been mentioned in the books, but EE brought them to life in second edition.
There are some differences between the Burning Blades and the Tribal Warriors. The Burning Blades have better stats ( RC, AR, Cause Fear and fire-based weapons ) than the Tribal Warriors but they come along with 42 points for a normal Warrior.
just my two cents...
Anomander Rake