It seems you are supposed to be able to throw a gnomish firebomb at a Bubbling Mass in HHK and that should do something to get it to move past being invulnerable, but when I tried to throw one it didn't do anything. I threw the item using a Druid (no throwing skill - don't know if that matters?) and it visibly threw at the bubbling mass and removed the firebomb from my inventory, but nothing happened.