There's some interesting ideas in here - thanks for sharing them! It's great to see the constructive discourse and reasoning behind these idea threads.
We are, indeed, redesigning the 6-Piece Set Bonus for Firebird's Finery. In this case, it's been completely redesigned, though we do want to keep Fire thematically appropriate to the set. We've removed the on-death qualifier from the set bonus, and dealing Fire damage will now cause enemies to start burning for the same amount of damage over 3 seconds, stacking up to a total of 3000% weapon damage as Fire per second. Once that cap is reached, enemies will instead burn until they die.
You'll be able to give this new set bonus a try in the next PTR patch, which we should be looking at arriving pretty soon. As always, be sure to let us know what you think when you give it a whirl!