Let me list the bugs:
New bugs (07/17 patch)
- Mirror images randomly remove firebirds stacks or prevent them from building up.
- Conflagrate doesn't count towards stacks at all against some mobs (mainly whites).
- Environment fire damage removes firebird's DoT and prevents you from building it up again.
Old bugs not yet fixed:
- Extra explosions from Wand of Woh don't apply stacks.
- Hydra doesn't apply stacks (at least this one is already listed in the known issues topic http://us.battle.net/d3/en/forum/topic/13242714346)
- Only 1 person in the party can have Firebird's DoT applied to a mob.
If no one's drawing attention to these issues, they will go unfixed in the 2.1 release, and we can say goodbye to the set.
I'll update this list with further bugs I find out, I'm sure there are many more.