I've been testing a bit with my Wizard and maybe found other things aswell:
1.) The Firebird Elite-Stack can also be triggered (and kept) when you kill a burning blue-pack one after one, as long as not all of them burn
2.) A burning Elite (Blue-Pack for me) can trigger just a normal burning stack like you ignited just one white mob (had this once, will try to find out how exactly)
3.An Elite (Yellow for me) can burn, and NOT give a stack at all.
I've been testing a bit with my Wizard and maybe found other things aswell:
1.) The Firebird Elite-Stack can also be triggered (and kept) when you kill a burning blue-pack one after one, as long as not all of them burn
2.) A burning Elite (Blue-Pack for me) can trigger just a normal burning stack like you ignited just one white mob (had this once, will try to find out how exactly)
3.An Elite (Yellow for me) can burn, and NOT give a stack at all.